ManagerHandlers should all be static

[Timidger]

From the conclusion of working on #238 it's apparent that there's never a need to have dynamic allocation for the manager handlers. The state can easily be stored in the Compositor state, which is allocated once at the start of the program, and you can only ever have one variant of the manager handlers since you don't want to manage the same resource twice.

The api should change to look like this (this is xdg_shell_v6_test translated over):

let input_manager = input::Manager::builder()
    .pointer_added(pointer_added)
    .keyboard_added(keyboard_added);
let output_manager = output::Manager::builder()
    .on_frame(on_frame);
let xdg_shell_v6_manager = xdg_shell_v6::Manager::builder()
    .destroyed(xdg_shell_v6_destroyed);
compositor::Builder::new().gles2(true)
        .wl_shm(true)
        .input_manager(input_manager)
        .output_manager(output_manager)
        .xdg_shell_v6_manager(xdg_shell_v6_manager)
        .build_auto(State::new(xcursor_manager, layout, cursor));