We are mixing bool and fancy 3-in-1 func-set buttons in the outliner.
So they would return different pushed state in
ui_drag_toggle_but_pushed_state().

We now have a callback function that allows the button to set its own
pushed_button_state callback function.

Note: This is a bit of overkill since we are planning to change the
3-in-1 outliner buttons. That said, it may be nice to have, since in the
future we can mix those buttons for other things.