This commit also takes the first step towards making it configurable;
override `internal/palettes.DefaultPaletteName` at build time to choose
one palette over another. It would be nice to set this at runtime!
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.
It's a complete mess for now - many things are out of place or shown
when they shouldn't be - and we can't move around the game map. But,
it's a good start.
Nothing is actually processed in this way yet, but there is a new
assertion forbidding certain types of records from having children.
Because of this new assertion, our menutype tweaks must be moved up a
layer into internal/menus. They fit better there anyway.
This commit implements loading and saving options from/to config, and
enough UI toolkit magic to allow changes to boolean options to be
persisted.
We now respect the "play movies" setting and take screen resolution
from the config file.
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.
