feat: option to exit window while retaining fullscreen (#516)

[nktnet1]

Describe your PR, what does it fix/add?

Resolves #516 by completing the second (final) point:

when a fullscreen window is being closed and it was not the only window on the current workspace, the next window that gains focus would become the new fullscreen window

This PR introduces a new misc option:

misc { 
    exit_window_retains_fullscreen = true
}

The implementation makes a backup of the current window's fullscreen mode & state, which is then applied to the next window candidate that will gain focus (if any).

Is there anything you want to mention? (unchecked code, possible bugs, found problems, breaking compatibility, etc.)

I think exit_window_retains_fullscreen is a suitable name and belongs in misc as a new variable, although I'm not too familiar with the code base and all available options yet.

Is it ready for merging, or does it need work?

Code-wise, I think it's ready.

Will need to make another pull request for hyprland-wiki if this PR is in the right direction (happy to write the documentation if this gets the green lights).

[vaxerski]

lgtm wiki mr needed

[nktnet1]

lgtm wiki mr needed

Documentation has been added here:

• docs: added variable misc:exit_window_retains_fullscreen hyprland-wiki#717

Thank you!