In addition to `button_count`, we keep track of the current buttons
pressed just as in `wlr_keyboard`.
Add `set_add` and `set_remove` to assist with this. These functions can
only be used with values greater than 0 (such as the button/key masks
for keyboards and pointers).
Partially addresses:
- https://github.com/swaywm/wlroots/issues/1716
- https://github.com/swaywm/wlroots/issues/1593
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