Create a new C++ example compositor

qwindow-compositor (which people often use as a reference for c++ compositors), use deprecated API (xdg-shell unstable v5) and support neither xdg-shell v6, nor stable. Upgrading it to xdg-shell v6 is also not ideal, as it was written with wl-shell in mind, then xdg-shell v5 was shoehorned in afterwards.

Create a new example with up-to-date code, based on minimal-cpp, instead, with xdg-shell v6 and or stable support and implement basic functionality, i.e. mouse, touch and keyboard input, raise, lower, move resize.