Move X11-specific KWindowSystem functions to new KX11Extras class
A lot of the fuctions in KWindowSystem are specific to X11 and have no Wayland/Windows/macOS counterpart.
Some are implemented on Windows/macOS, but are often not relevant there since they are only really relevant for Plasma
For the API user of KWindowSystem this is a problem because it is unclear what functions are expected to work on which platforms
To solve this introduce a new class KX11Extras to hold those functions that only make sense/are supported on X11