API: Deprecate gtk_container_set_reallocate_redraws()
GtkAssistant: Avoid a crash

The child properties in GtkAssistant are somewhat broken, since
they are not on direct children - but that is no reason to crash
if somebody does ask for child properties of direct children.
inspector: redo property editing

Move away from cell editing, and use a popover instead. This makes
it easier to e.g. use a color chooser - there's just not enough room
in a cell for many things.

Much of this code is adapted from tests/prop-editor.c.