Redesign date/time widget in AnnotationDialog.
DateBar: automatically adjust range to images.
Allow scrolling of DateBar using middle mouse button.
DateBar: don't do autorange if lower date is null.
DateBar/autorange: remove spurious redraws.
Add WhatsThis info for DateBar.
Better text for DateBar help.
AnnotationDialog/Dialog: remove unneccessary variable.
Swallow Return key in CompletableLineEdit.

Fixes a bug that was exposed by the recent AnnotationDialog date picker
changes (bf3e9fa): Pressing Enter in the lineEdit would also trigger the
"previous image"-button.
Crazy/i18ncheckarg: fix XMLDB/XmlReader.cpp.
AsyncLoader: only search icon for broken file once.
Fix compile issue in OS/X.

BUG: 313987
Fix crash when member group category doesn't exist.

When the category of the member-group does not exist, a warning is
printed on the console and a matching category is created.

CCBUG: 319135
Don't escape strings for xml attribute values.

BUG: 319135
Video filenames must be unique for HTML generation
Remove unused includes.
Review locking in AsyncLoader.

Add some comments, no raw access to lock (for readability), make _lock
mutable.