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
Merge pull request #234 from ControlSystemStudio/bug227_zoom_when_scrolling_gets_disabled

Bug227 zoom when scrolling gets disabled: resolves  #227