X11 support roadmap about glutin

@tomaka
I've added fullscreen support for X11 (see my branch x11_fullscreen).
A few issues are still remaining (that's why I didn't do a PR):

• When exiting fullscreen mode, the screen doesn't switch back to its native resolution
• To access the monitor, we need to have access to the display, which means that it can't be used with WindowBuilder, as it is currently with Windows. Thus, refactoring needs to be done on that part, but I don't have access to a windows machine at the moment, and maybe you already have an idea about how to do it properly. I've currently put some dummy values so that we can use the fullscreen.rs example.

You can also go through my code and give me any feedback, as this is the first code I write in rust.

[tomaka]

Nice!

My initial plan was that MonitorID was some kind of "system ID" which doesn't change even if you close and restart the program (it can be an integer, a string, etc. but not a pointer).
When you call get_available_monitors or get_primary_monitor, a display is created, the monitors are retreived, and the display is closed.
Then when you create the window, the display is reopened, and fail! is called if the MonitorID can't be found in the new display.

However I don't know if that's good practice. An alternative would be to use a static storage for the Display, like glfw does.

Also, why add is_fullscreen to the WindowBuilder instead of just calling monitor.is_some()?

Ok,
I could try to implement the first solution (create display, close it, and reopen it) and see how things go.
For is_fullscreen, I'll use monitor.is_some() instead.
I won't be able to work on it before two weeks though, as I'm on holidays.