To do this, MENU and SUBMENU are split into two types (at last), and
a Widget type is introduced. This should allow lots of code to be
removed at some point.
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.
This cuts memory use significantly, since many sprites in an object are
never used. We can get savings over time by evicting sprites when they
go out of scope, but that's, well, out of scope.
To achieve this, I introduce an assetstore package that is in charge of
loading things from the filesystem. This also allows some lingering
case-sensitivity issues to be handled cleanly.
I'd hoped that creating fewer ebiten.Image instances would help CPU
usage, but that doesn't seem to be the case.
