Wire up inventory select to ship state

Also mixed into this commit:

* Use returning drivers where possible
* Make the credits screen returnable via click

internal/flow/choices.go

@@ -0,0 +1,20 @@
+package flow
+
+func (f *Flow) linkChoices() {
+	f.onClick(choices, "2.1", f.setReturningDriver(choices, loadGame)) // Load another game button
+	f.onClick(choices, "2.2", f.setReturningDriver(choices, saveGame)) // Save this game button
+	f.onClick(choices, "2.3", f.setReturningDriver(choices, options))  // More options button
+	f.onClick(choices, "2.4", func() {                                 // New Game button. FIXME: should ask about the emperor
+		f.ship.Reset() // Throws away in-progress game
+		f.reset()
+	})
+
+	f.onClick(choices, "2.5", f.setReturningDriver(choices, credits)) // Credits button
+	f.onClick(choices, "2.6", f.setExit)                              // Quit button. FIXME: should ask about the emperor
+	f.onClick(choices, "2.7", f.returnToLastDriver(choices))          // Back button
+
+	// loadGame is linked by main
+	f.linkSaveGame()
+	// options is linked by main
+	f.linkCredits()
+}