Handle keyboard and button state in X11 events.

[jneem]

Fixes #893.

[xStrom]

Generally looks good.

The helper methods mouse_button / mouse_buttons / key_mods don't really need to be methods though, they don't use self. Let's make them associated functions for example by removing the &self paramater and then calling them as Window::mouse_buttons(....). Alternatively can just move them into the module scope, e.g. to the bottom of the file.

[jneem]

Thanks for the review! I've moved the functions in question to the module scope.

[xStrom]

Yeah that looks better. Need a rebase with master and then this will be ready for merge.

[xStrom]

Thanks!