Improve documentation around "visibility" and "occlusion"

[daxpedda]

WindowEvent::Occluded says:

This is different to window visibility as it depends on whether the window is closed, minimised, set invisible, or fully occluded by another window.

Window::is_visible() says:

Gets the window's current visibility state.

Even though WindowEvent::Occluded differentiates itself from "window visibility", "window visibility" itself is not really explained anywhere. Probably the mention of "set invisible" in Occluded doesn't help either.

[daxpedda]

There was a brief discussion in IRC about adding a Window::is_occluded(), but I'm really not sure this makes sense because of the ambiguity around "Window::is_visible()`.

Additionally with #2929, we have another ambiguity: WindowState, if the window is minimized, is it also Occluded? I'm just not sure if it makes sense to split up like that.

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Just some quick not really thought through suggestion:

Maybe WindowState should also include "window visibility" and maybe we should also remove is_maximized() and all these other methods and just expose Window::window_state(), then we could have one event: WindowEvent::Configured that handles all states and one method Window::window_state() that can query all of them as well (maybe Window::set_window_state() as well?).

The WindowEvent::Occluded event should always fire if the window isn't visible, and with visible I mean if the user can see the window or not, period. Then a Window::is_occluded() makes sense and would also be true if minimized or doesn't have "window visibility".