borderless fullscreen window support for x11

This adds borderless fullscreen window support for x11.
Also moves the disable resize x11 code from context_gl_x11.cpp to os_x11.cpp.

I tested on Gnome3. alt+tab works.
I tested on Openbox standalone and xfce4. alt+tab works.
I tested on xfce4. alt+tab works in projects that don't set mouse to captured in gdscript.
Doesn't look like Cinnamon is completely in Debian Testing yet (or I'm doing something wrong), and I don't run Ubuntu, so I can't test Unity.