api: make input focus global state

[ammen99]

The keyboard input focus has always been a global state (to be precise, per-seat, but Wayfire has only one seat anyway). However, previously the focused view has been managed per-output, which makes little sense and does not reflect reality, nor the desired behavior. This commit moves all keyboard-focus functionality out of wf::output_t and into wf::seat_t.

Fixes #1892

[ammen99]

Breaking changes for plugins: focus is no longer per-output, hence:

1. wf::get_core().get_active_output() and wf::get_core().focus_output() have been moved to wf::seat_t (so for example use wf::get_core().seat->focus_output() to focus an output).
2. output->refocus() becomes wf::get_core().seat->refocus().
3. output->get_active_view() can be replaced with wf::get_active_view_for_output(output) or wf::get_core().seat->get_active_view(). The latter returns the globally focused view, the former checks whether the output is focused, if not, it returns null, otherwise the same as the seat's get_active_view().
4. output->focus_view() is not a very well defined function, because it does a ton of things. There are multiple alternatives available, depending on what one wants to do:
   • wf::get_core().seat->focus_view(): this is a very basic function and just focuses the view, no unminimizing, raising the view, etc. This is a rather low-level function.
   • wf::get_core().default_wm->focus_raise_view(): this is pretty much calling output->focus_view(view, true): focus the view, raise to the top, unminimize, etc.
   • wf::get_core().default_wm->focus_request(): this is the same as focus_raise_view(), but it emits a focus_request signal, which gives plugins a chance to deny the focus request. This is used for example when a new view is mapped, so that focus stealing prevention can work.
   • There is no output-specific view-focused signal, instead, plugins can use the global keyboard focus changed in core to figure out what is going on with focus. Also, the pre-focus signal has been removed in favor of the focus-request as explained in the previous point.