textview: Call gtk_widget_size_allocate() on children widgets invariably

textview: Call gtk_widget_size_allocate() on children widgets invariably

There was this hack, taken verbatim from GtkCList according to the comment,
that would recursively translate the allocation during scrolling, and set
it on children widgets through the direct gtk_widget_set_allocation() setter.

Since commit 4f89eb05cf8a, this has caused the wrong clipping areas to children
widgets of a textview. The reasons for this seem lost in time, and the approach
seems indeed wrong for windowed widgets as the repositioning of those windows
couldn't happen.

So replace all of this with just a gtk_widget_size_allocate() call, which does
work ok for the children widgets embedded in the "multiple views" gtk demo, and
ought to work for every other widget.

https://bugzilla.gnome.org/show_bug.cgi?id=732900