xwayland: add a setting to disable selected X extensions

The X server, including Xwayland, can be compiled with different X11 extensions enabled at build time.

When an X11 extension is built in the X server, it's usually also enabled at run time. Users can chose to disable those extensions at run time using the X server command line option "-extension".

However, in the case of Xwayland, it is spawned automatically by the Wayland compositor, and the command line options are not configurable by users.

Add a new setting to disable a selected set of X extension in Xwayland at startup, without needing to rebuild Xwayland.

Of course, if Xwayland is not built with a given extension support in the first place (which is the default for the security extension for example), that option has no effect.