This way, we no longer prescribe the use of either GtkListItem or
GtkListItemWidget.
This means we can use it in other places, such as for custom section
header objects or with my Canvas ideas.
Match GtkListBox, so it's possible to use the same styles for them.
Update GtkListView and GtkGridView docs to reflect that, fix a few gtk-doc
formatting leftovers along the way.
Add a mode to GtkListItemWidget that activates on
single click and selects on hover. Make
GtkListItemManager set this on its items
when its own 'property' of the same name is set.
We only create them in root/unroot (they should be created in
appear/disappear, but that vfunc doesn't exist yet), that way we can
avoid expensive work while the widget isn't used for anything.
Instead of 6 vfuncs, we now have 3 and rely on the factory keeping track
of what it needs to do.
We're doing lots of dancing from one object to another here, but this
will hopefully get simpler with further commits.
This splits GtkListItem into 2 parts:
1. GtkListItem
This is purely a GObject with public API for developers who want to
populate lists. There is no chance to cause conflict with GtkWidget
properties that the list implementation assumed control over and
defines a clear boundary.
2. GtkListItemWidget
The widget part of the listitem. This is not only fully in control of
the list machinery, the machinery can also use different widget
implementations for different list widgets like I inted to for
GtkColumnView.
