Device classes split out of Seat class

[romangg]

The Wrapland::Server::Seat class is too large, a lot of functionality is crammed into it that could instead be split out into new free functions or classes in separate files.

This holds in particular for everything about the common input devices:

• wl_keyboard
• wl_pointer

Instead for such devices separate classes could be introduced that are created by the compositor once per physical device. All state and resource tracking then happens in these classes instead of the Seat class.

This means in detail for example for keyboards:

• Rename Wrapland::Server::Keyboard to Wrapland::Server::keyboard_res.
• Create a new class Wrapland::Server::keyboard which is created by the compositor representing the single keyboard input scheme of a Seat.
• Associate that new keyboard class with a Seat (either directly in ctor, factory function or worse a separate member function call).
• Calls like get_keyboard are executed in the new keyboard class.
• The keys struct in the Seat is gone.

[romangg]

mentioned in commit dfa699d57a3945edf8d341e19b91a85977365916