Setting test containers' content to UI_Labels
Giving all UI containers their own WINDOW

Previously they all drew to the same window and handled borders
logically instead of with the help of ncurses' WINDOW interface. Don't
know why I did it that way to begin with, this is much easier.
Adding checks for containers with no children

Avoids potential segfault
Checking for UI_Containers with not-yet-created WINDOWs

Avoids another potential segfault
Adding rect_intersection()

Returns the area of intersection between two struct rects
Finishing implementation of UI_GameView element

Provides the window from the UI system into the gameworld. Currently
hardcoded to center on the player, should eventually be changed to refer
to some more generic value (which depends on the player most of the
time)
Adding some comments to UI_DrawGameView()

It's a monster, it kind of needs them.