[ModeButton] n_items is not correct when called from on_mode_removed callback

If you connect mode_removed signal to a callback and try to obtain the count of items from it, the number is not correct. In my opinion the problem is in the remove method, where the mode_removed signal shouldn't be emitted before item_map.unset (index).

You can compile and run the example with:
valac --pkg=granite --pkg=gtk+-3.0 example.vala && ./example

take look at the terminal output.