Display MainGame.mnu and map in ordoor simultaneously

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.

internal/flow/drivers.go

@@ -0,0 +1,74 @@
+package flow
+
+import (
+	"fmt"
+)
+
+type driverName string
+
+const (
+	// Names of all the drivers
+	main      driverName = "Main"
+	levelPly  driverName = "LevelPly"
+	singles   driverName = "Singles"
+	randomMap driverName = "RandomMap"
+	newGame   driverName = "NewGame"
+	loadGame  driverName = "LoadGame"
+	options   driverName = "Options"
+	kbd       driverName = "Keyboard"
+	bridge    driverName = "Bridge"
+	briefing  driverName = "Briefing"
+	choices   driverName = "Choices"
+	saveGame  driverName = "SaveGame"
+	credits   driverName = "Credits"
+	arrange   driverName = "Arrange"
+
+	configureUltEquip      driverName = "Configure_UltEquip"
+	configureVehiclesUltra driverName = "Configure_Vehicles_Ultra"
+
+	mainGame driverName = "MainGame"
+)
+
+var (
+	driverNames = []driverName{
+		main, levelPly, singles, randomMap, newGame, loadGame, options, kbd,
+		bridge, briefing, choices, saveGame, credits, arrange,
+		configureUltEquip, configureVehiclesUltra,
+		mainGame,
+	}
+)
+
+// from is the child menu, to is the parent
+func (f *Flow) returnToLastDriverNow(from driverName) error {
+	to, ok := f.returns[from]
+	if !ok {
+		return fmt.Errorf("Couldn't work out where to return to from %v", from)
+	}
+
+	delete(f.returns, from)
+
+	f.setDriverNow(to)
+	return nil
+}
+
+func (f *Flow) setDriver(name driverName) func() {
+	return func() {
+		f.setDriverNow(name)
+	}
+}
+
+func (f *Flow) setDriverNow(name driverName) {
+	f.current = f.drivers[name]
+}
+
+// from is the parent menu, to is the child
+func (f *Flow) setReturningDriver(from, to driverName) func() {
+	return func() {
+		f.setReturningDriverNow(from, to)
+	}
+}
+
+func (f *Flow) setReturningDriverNow(from, to driverName) {
+	f.returns[to] = from
+	f.setDriverNow(to)
+}