Simplify DND checkboxes drawing, this also fixes incorrect drawing on
Mac when file tree widget is inactive.
Do better job calculating column widths for file tree to avoid ellipsis.
Fix file tree sorting order for size and priority columns.
Change key to toggle priorities to Shift+Space instead of Enter/Return
to avoid conflicts with name editing and default button handling.
Fix selected tracker item background drawing in certain cases.
Split FileTree.{h,cc} and FilterBar.{h,cc} files so that each class
is in its own file.
This breaks translations (some classes got renamed => context changed),
to be fixed by next commit (along with Tx sync).
Mike Gelfand