When moving a descendant of a tabbed or stacked container, it is possible
for the target node to be the node being moved. This causes a segfault in
`handle_finish` since the node will be detached and then attempted to be
attached to it own parent, which is NULL due to the detach. In this
case, the target node should not be set to the node being moved, but the
parent of the node. This also allows for a descendant of a tabbed or
stacked container to be dragged out of the tabs/stacks and to be a
sibling of the tabbbed/stacked container, which was not previously
possible.
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
