Added minimum size parameter to GtkWidgetClass->adjust_size_allocation.

Added minimum size parameter to GtkWidgetClass->adjust_size_allocation.

This allows us to add a check before executing
->get_preferred_height_for_width() to ensure we always
request for at least the minimum required size (and lets
us remove the warning in gtkcontainer.c telling implementors
to do this check manually from thier container implementations).