Add a widget gallery page

This came up in https://gitlab.gnome.org/GNOME/pygobject/issues/208

Add a list of all non-abstract Gtk.Widget subclasses and show an
example image for each of them.
The images are taken from the gtk+ documentation.

The goal is that users can quickly get an overview of the available widgets
and to make it possible to find a widget type without knowing the name.

Author: lazka

source/gallery.txt

@@ -0,0 +1,63 @@
+Widget Gallery
+==============
+
+=================================== ======
+:class:`Gtk.AboutDialog`            .. image:: widgets/aboutdialog.png
+:class:`Gtk.AccelLabel`             .. image:: widgets/accel-label.png
+:class:`Gtk.ActionBar`              .. image:: widgets/action-bar.png
+:class:`Gtk.AppChooserButton`       .. image:: widgets/appchooserbutton.png
+:class:`Gtk.AppChooserDialog`       .. image:: widgets/appchooserdialog.png
+:class:`Gtk.Assistant`              .. image:: widgets/assistant.png
+:class:`Gtk.Button`                 .. image:: widgets/button.png
+:class:`Gtk.CheckButton`            .. image:: widgets/check-button.png
+:class:`Gtk.ColorButton`            .. image:: widgets/color-button.png
+:class:`Gtk.ColorChooserDialog`     .. image:: widgets/colorchooser.png
+:class:`Gtk.ComboBox`               .. image:: widgets/combo-box.png
+:class:`Gtk.ComboBoxText`           .. image:: widgets/combo-box-text.png
+:class:`Gtk.Entry`                  .. image:: widgets/entry.png
+:class:`Gtk.FileChooserButton`      .. image:: widgets/file-button.png
+:class:`Gtk.FileChooserDialog`      .. image:: widgets/filechooser.png
+:class:`Gtk.FlowBox`                .. image:: widgets/flow-box.png
+:class:`Gtk.FontButton`             .. image:: widgets/font-button.png
+:class:`Gtk.FontChooserDialog`      .. image:: widgets/fontchooser.png
+:class:`Gtk.Frame`                  .. image:: widgets/frame.png
+:class:`Gtk.GLArea`                 .. image:: widgets/glarea.png
+:class:`Gtk.Grid`                   .. image:: widgets/grid-packing.png
+:class:`Gtk.HeaderBar`              .. image:: widgets/headerbar.png
+:class:`Gtk.IconView`               .. image:: widgets/icon-view.png
+:class:`Gtk.Image`                  .. image:: widgets/image.png
+:class:`Gtk.InfoBar`                .. image:: widgets/info-bar.png
+:class:`Gtk.Label`                  .. image:: widgets/label.png
+:class:`Gtk.LevelBar`               .. image:: widgets/levelbar.png
+:class:`Gtk.LinkButton`             .. image:: widgets/link-button.png
+:class:`Gtk.ListBox`                .. image:: widgets/list-box.png
+:class:`Gtk.LockButton`             .. image:: widgets/lockbutton.png
+:class:`Gtk.MenuBar`                .. image:: widgets/menubar.png
+:class:`Gtk.MenuButton`             .. image:: widgets/menu-button.png
+:class:`Gtk.MessageDialog`          .. image:: widgets/messagedialog.png
+:class:`Gtk.Notebook`               .. image:: widgets/notebook.png
+:class:`Gtk.Paned`                  .. image:: widgets/panes.png
+:class:`Gtk.PlacesSidebar`          .. image:: widgets/placessidebar.png
+:class:`Gtk.ProgressBar`            .. image:: widgets/progressbar.png
+:class:`Gtk.RadioButton`            .. image:: widgets/radio-group.png
+:class:`Gtk.RecentChooserDialog`    .. image:: widgets/recentchooserdialog.png
+:class:`Gtk.Scale`                  .. image:: widgets/scales.png
+:class:`Gtk.Scrollbar`              .. image:: widgets/scrollbar.png
+:class:`Gtk.ScrolledWindow`         .. image:: widgets/scrolledwindow.png
+:class:`Gtk.SearchBar`              .. image:: widgets/search-bar.png
+:class:`Gtk.SearchEntry`            .. image:: widgets/search-entry.png
+:class:`Gtk.Separator`              .. image:: widgets/separator.png
+:class:`Gtk.SpinButton`             .. image:: widgets/spinbutton.png
+:class:`Gtk.Spinner`                .. image:: widgets/spinner.png
+:class:`Gtk.Stack`                  .. image:: widgets/stack.png
+:class:`Gtk.StackSwitcher`          .. image:: widgets/stackswitcher.png
+:class:`Gtk.Statusbar`              .. image:: widgets/statusbar.png
+:class:`Gtk.Switch`                 .. image:: widgets/switch.png
+:class:`Gtk.TextView`               .. image:: widgets/multiline-text.png
+:class:`Gtk.ToggleButton`           .. image:: widgets/toggle-button.png
+:class:`Gtk.ToolPalette`            .. image:: widgets/toolpalette.png
+:class:`Gtk.Toolbar`                .. image:: widgets/toolbar.png
+:class:`Gtk.TreeView`               .. image:: widgets/list-and-tree.png
+:class:`Gtk.VolumeButton`           .. image:: widgets/volumebutton.png
+:class:`Gtk.Window`                 .. image:: widgets/window.png
+=================================== ======

source/index.txt

@@ -38,6 +38,7 @@ Contents:
    introduction
    basics
    unicode
+   gallery
    layout
    label
    entry