Slightly rework clip handling

Slightly rework clip handling

always initialize clips to the (content) allocation, don't walk up the
widget hierarchy in gtk_widget_set_clip, implement
gtk_widget_size_allocate in GtkSeparator. This way we don't end up using
uninitialized clip values.

The entire clip handling is up for major rework since we can't and don't
want to force every single widget to call _set_clip in size-allocate
implementations.