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/ship/ship.go

@@ -1,8 +1,18 @@
 package ship
 
+type DifficultyLevel int
+
+const (
+	DifficultyLevelMarine  DifficultyLevel = 0
+	DifficultyLevelVeteran DifficultyLevel = 1
+	DifficultyLevelHero    DifficultyLevel = 2
+	DifficultyLevelMighty  DifficultyLevel = 3
+)
+
 // Ship encapsulates campaign state, including current location in the campaign,
 // marines and their stats, supplies, etc.
 type Ship struct {
+	Difficulty   DifficultyLevel
 	NextScenario string
 
 	Squads      []*Squad
@@ -68,3 +78,16 @@ type Honours struct {
 	PuritySeal     bool
 	ImperialLaurel bool
 }
+
+func New() *Ship {
+	s := &Ship{}
+	s.Reset()
+
+	return s
+}
+
+func (s *Ship) Reset() {
+	*s = Ship{
+		Difficulty: DifficultyLevelVeteran, // Default difficulty level
+	}
+}