`WarHammer.ani` turns out to be a regular `obj` file; `WarHammer.idx`
is partially decoded, but I'm struggling to link it to the former in
a reasonable way at the moment.
With this commit, we get a ui.Interface and ui.Widget type. The
interface monitors hover and mouse click state and tells the widgets
about them; the widgets execute code specified by the application when
events occur.
Next step: have wh40k load the main menu and play sound, etc.
