widget: Drop the ::size-allocate signal

widget: Drop the ::size-allocate signal

This signal is mainly used for bad things, and we
don't want to emit signals during layout if we can
avoid it.

If you are subclassing, you should either use a
layout manager or override the size_allocate vfunc.

If you are using a GtkDrawingArea or GtkGLArea,
use their ::resize signals to learn about size
changes.

Fixes: #2705