This splits each seat operation (drag/move tiling/floating etc) into a
separate file and introduces a struct sway_seatop_impl to abstract the
operation.
The move_tiling_threshold operation has been merged into move_tiling.
The main logic for each operation is untouched aside from variable
renames.
The following previously-static functions have been made public:
* node_at_coords
* container_raise_floating
* render_rect
* premultiply_alpha
* scale_box
