This introduces seat_get_focus_inactive_tiling and updates
`focus mode_toggle` to use it instead, because the previous method
wasn't guaranteed to return a tiling view.
Also does a few other related things:
* Now uses enum wlr_edges instead of our own enum resize_edge
* Now uses wlr_xcursor_get_resize_name and removes our own
find_resize_edge_name
* Renames drag to move for consistency
This implements the following:
* `floating_modifier` configuration directive
* Drag a floating window by its title bar
* Hold mod + drag a floating window from anywhere
* Resize a floating view by dragging the border
* Resize a floating view by holding mod and right clicking anywhere on
the view
* Resize a floating view and keep aspect ratio by holding shift while
resizing using either method
* Mouse cursor turns into resize when hovering floating border or corner
Rather than maintain copies of the entire focus stack, this PR
transactionises the focus by introducing two new properties to the
container state and using those when rendering.
* `bool focused` means this container has actual focus. Only one
container should have this equalling true in its current state.
* `struct sway_container *focus_inactive_child` points to the immediate
child that was most recently focused (eg. for tabbed and stacked
containers).
