pb-timeline selection label is wrong

[DrRataplan]

To repro:

• Visit https://koenigsfelden.sources-online.org/index.html?query=&sort=date&subtype=edition&subtype=title&subtype=regest&subtype=comment&subtype=notes&dates=&start=1
• Observe the label (Angezeigter Zeitraum: 1270 – 1655)
• Go to the last item in the timeline: 1655 - 1659 : 1
• Actually click that item
• Check the date of that single document: 01. Februar 1659

Expected:

• The label should have been 1270 - 1659

Actual:

• The label of the pb-timeline component indicates 1270 – 1655, whichis wrong

The fix is to use the selectionStart / selectionEnd fields in the data of the pb-timeline. PR incoming.