Simplified and repaired resizing of columns inside CustomListBox

Extracted resize functionality into ColumnResizing type. Fixed strange glitches when columns are resized.
cleanup after merging pull request
DiffView: fixed separator color in split view mode
create branch dialog: fixed default local branch name when creating from remote branch with multi-part name (closes #3)

Old behavior: remote/feature/foo -> foo
New behavior: remote/feature/foo -> feature/foo