Add margin and alignment properties to GtkWidget

Add margin and alignment properties to GtkWidget

h-align = START,END,CENTER,FILL
v-align = START,END,CENTER,FILL
margin-left,right,top,bottom
margin

These should obsolete all such similar properties on
layout containers, GtkMisc, GtkAlignment, GtkContainer::border-width

Margin is outside the size request.
If margin were not outside the set_size_request() it would not work the
same way as container-supplied (child property) padding.

Conceptually set_size_request() forces the value from the subclass
(the original unadjusted request) and then we go on to adjust
the request further by adding the margin.