window-x11: Remove a use of meta_window_get_position
keybindings: Fix code style
keybindings: Correct move_to_corner_backend

meta_window_get_position() returns the client rect position, which
we then pass to meta_window_move_frame. Just use the existing frame
rect coordinates.
window-wayland: Don't use meta_window_get_position
keybindings: Replace the incomprehensible set of flags with a gravity param