Start work on menu interactivity.

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.

internal/assetstore/assetstore.go

@@ -33,6 +33,7 @@ type AssetStore struct {
 
 	// These members are used to store things we've already loaded
 	maps   map[string]*Map
+	menus  map[string]*Menu
 	objs   map[string]*Object
 	sets   map[string]*Set
 	sounds map[string]*Sound
@@ -81,6 +82,7 @@ func (a *AssetStore) Refresh() error {
 	// Refresh
 	a.entries = newEntryMap
 	a.maps = make(map[string]*Map)
+	a.menus = make(map[string]*Menu)
 	a.objs = make(map[string]*Object)
 	a.sets = make(map[string]*Set)
 	a.sounds = make(map[string]*Sound)
@@ -89,7 +91,12 @@ func (a *AssetStore) Refresh() error {
 }
 
 func (a *AssetStore) lookup(name, ext string, dirs ...string) (string, error) {
-	filename := canonical(name + "." + ext)
+	var filename string
+	if ext != "" {
+		filename = canonical(name + "." + ext)
+	} else {
+		filename = canonical(name)
+	}
 
 	for _, dir := range dirs {
 		dir = canonical(dir)