window: Separate pointer-focus operations and explicit-focus operations
Some clients, like on-screen keyboards, don't want their windows to be
activated on click, as that would steal focus from the window they're
trying to send events to. They do this by setting the Input Hint in
WM_HINTS to be FALSE, along with not specifying WM_TAKE_FOCUS in their
WM_PROTOCOLS.
However, in this case, when a window tries to get focus in this scenario,
we focus the frame so that a11y and key navigation works properly.
Both policies aren't acceptable -- we can't make OSK steal focus from
windows on click, and we can't make an OSK un-Alt-Tabbable-to.
To solve this, split meta_window_focus() into two different focus policies:
* meta_window_focus_implicitly() should be called on pointer click or
focus-follows-mouse or similar cases where we may not want to forcibly
set focus for clients that don't want it.
* meta_window_focus_explicitly() should be called by pagers, like
the gnome-shell overview, or Alt-Tab. In this case, most of the existing
clients are using meta_window_activate(), so simply adapting that so
it calls meta_window_focus_explicitly() should be enough.
cullable: Turn cull_out / reset_culling into a separate interface
Instead of hardcoded knowledge of certain classes in MetaWindowGroup,
create a generic interface that all actors can implement to get parts of
their regions culled out during redraw, without needing any special
knowledge of how to handle a specific actor.
The names now are a bit suspect. MetaBackgroundGroup is a simple
MetaCullable that knows how to cull children, and MetaWindowGroup is the
"toplevel" cullable that computes the initial two regions. A future
cleanup here could be to merge MetaWindowGroup / MetaBackgroundGroup so
that we only have a generic MetaSimpleCullable, and move the "toplevel"
cullability to be a MetaCullableToplevel.
