qtutils: add hide_button_menu_indicator()

Signed-off-by: David Aguilar <davvid@gmail.com>
commitmsg: hide the menu indicator

Generalize qtutils.hide_button_menu_indicator() and use it for the
"Actions" menu button in the commit message editor.

Signed-off-by: David Aguilar <davvid@gmail.com>
diff: move the diff options menu into the DiffWidget

Refactor the code so that the diff options menu lives with the widget.

Signed-off-by: David Aguilar <davvid@gmail.com>