The previous implementation of focus handling assumed that only views can be
focused. Containers can also be focused with a command like `focus parent` or
`focus child`.
Change `set_focused_container()` to handle the case of the given container
being a container with children and update borders accordingly.
When titlebar is hidden, top border of the topmost view inside
tabbed/stacked container will not be drawn. This is changed in layout.c
On the other hand, top border should be drawn sometimes, for example
when titlebar is hidden on a view that is not the topmost inside
tabbed/stacked container. This is changed in border.c
Whenever a stacked or tabbed container has focused, paint the titlebars of all
its child windows to be focused as well to indicate the parent stack/tab
container has the focus.
This patch makes sure to clear the border buffer of fullscreen view so
the border doesn't get drawn behind a fullscreen view, which would be
visible if the view was transparent.
The borders are implemented as a surface/buffer attached to each view
which is sent to and rendered by wlc in the view_pre_render callback.
All the drawing logic is handled in sway/border.c and all the logic for
calculating the geometry of the border/view is handled in
`update_geometry` in sway/layout.c (same place as gaps are calculated).
