Data Browser: Fix zoom bug

When scrolling, then zooming into 'left' area of plot such that the
scrolling gets disabled because 'end' is no longer close to 'now', the
start/end times were wrong, using start of the zoomed area as the end.
Data Browser: Update changelog, remove debug printouts

Conflicts:
	applications/plugins/org.csstudio.trends.databrowser2/ChangeLog.html