Completely removed requisition cache from GtkWidget instance structure.

Completely removed requisition cache from GtkWidget instance structure.

Since we have a new mechanism for requesting sizes: GtkSizeRequestIface;
it makes no sense to maintain this cache on the GtkWidget structure...
removing the requisition cache however does not break the old "size-request"
signal which is there for backwards compatability reasons.

In any case widget->requisition should not have been accessed,
gtk_widget_get_child_requisition() would have been the correct way
to consult the cache.

This commit also deprecates the newly added gtk_widget_get_requisition()
API and makes it fallback on gtk_size_request_get_size().