Added Infinity Indicator

If a timeline end date is set to infinity then this is now shown by a
thin dotted line on the right of the timeline. This infinity dotted line
is weaker than the dotted line for an approximate date, which has longer
marks.
Merge branch 'infinity-indicator' of https://github.com/NeilCresswell/ChronoZoom into NeilCresswell-infinity-indicator
Merge branch 'NeilCresswell-infinity-indicator'