Wayland support by forcing x11

[almarklein]

Deals with #92. Probably won't entirely close it yet.

Changes

• Change canvas.get_window_id() into canvas.get_surface_info() to package all info in one dict, so that the get_display_id() method can be removed, and we can add info on the platform that the GUI toolkit uses (x11/wayland).
• New gui._gui_utils module to put some of this logic in.
• Also moved some utlils from gui.base to the gui utils module.
• Make it work for GLFW, by forcing x11.
• Make it work for Qt, by forcing x11.
• Make it work for wx, by forcing x11 (untested).
• Make sure the docs are correct.
• Add notes to the top post of Linux Wayland support #92.

Notes:

• This PR could be smaller if it'd just force x11 for glfw and qt, but in the process of trying out things, I also refactored a bit. These extra changes do make the code more "future ready" for when Wayland is supported better.
• Not sure what happens on more special linuxes like Raspberry Pi.

[almarklein]

I pushed new commits to this branch but they don't show up here.

edit: they are visible now