Added Expand Modes
Added set_expand function with options of "none" "outside" or "inside" modes.
The "inside" mode is the default and will result in the original behavior. The
main benefit is being able to actually center a widget in the available space
with options of how to draw the outside widgets (expand to take the space,
or not.) Further functionality can be had by ommiting one of the outside
widgets. Set default layout mode in the constructor.
Signed-off-by: Uli Schlachter <psychon@znc.in>