Removing an item from the ModeButton widget renders consecutive item indexes invalid

[Victor Martinez]

The current implementation of ModeButton.remove uses Gtk.Widget.destroy to remove an item from the widget. Since the widget's children list is obviously altered after that, the remove() method invalidates the indexes of the items that were following the index which was removed (e.g. if the removed item had index 4, then the current item 5 will be 4, and similarly for the rest of consecutive items).

Test attached (compile with vala 0.18 to get useful assertion messages).

[Victor Martinez (victored)]

The linked branch fixes the problem. It passes the test attached to the bug report.