Add XInput2 events support

This adds support for XInput2 input events and for actions to bypass grabs.

Bypassing grabs and thus being able to trigger the actions inside menus is very useful for several plugins affecting the screen or window looks for accessibility purposes, like zoom, contrast or color correction. The most obvious use case is zooming in and out when navigating menus, which a person with disabilities and relying on the zoom feature might require to browse items. Additionally, not being able to zoom inside menus is counter-intuitive and leads to user frustration.

Bypassing grabs is opt-in in a plugin's source code because while it has important use cases as mentioned above, it also has the important side effect of allowing the action to be triggered while the screen is locked, as screen lock is based on grabs. It is handy for accessibility effects that are also useful there, but allows for more user interaction on a locked screen. Thus, grab bypassing should only be opted in for actions with safe behavior (color inversion, zoom, …) and useful in grab contexts (menus, xlock, …), and NOT for rather dangerous actions (commands, window opening/closing, …) or more generally if not useful in grabbed contexts.

A following MR (in plugins repositories) will enable grabs bypassing for some selected modules.