VBoxLayout: Implement support for UISpacerItem layout items

* UISpacerItem: Rename ::change_size to ::set_size, add ::size getter.

* VBoxLayout: Introduce constructor for initializing spacing.

* VBoxLayout: Fix the underlying calculations used in the boundary
calculations involving the class spacing variable; this necessarily had
to be changed in order to support this addition.

VBoxLayout: Change default spacing between layout items to one (1):
a) standard behavior of many GUI sizers/layouts; b) appears to be more
friendly with using decorators on each layout item.
VBoxLayout: Implement more accurate minimum_size layout calculation

* VBoxLayout: Rename ::insert_widget to ::append_widget.
* VBoxLayout: Use 'for' loops instead of 'while' loops.

* Decorator: Implement getter & setter method for border frame bounds
* FinalFantasyFrame: Use drawables_ object's update flagging instead of
private boolean updated_ var.