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/save_game.go

@@ -0,0 +1,10 @@
+package flow
+
+func (f *Flow) linkSaveGame() {
+	// Save game button is disabled unless a listbox item is selected
+	// 3.2 is a hypertext that should be displayed when 3.1 is disabled... but
+	// it has no DESC.
+	f.setFreeze(saveGame, "3.1", true)
+	f.onClick(saveGame, "3.1", func() {})                      // TODO: Save Game button
+	f.onClick(saveGame, "3.3", f.returnToLastDriver(saveGame)) // Back button
+}