Enforce single-item selection in various actions

We want to show an error when the user tries to invoke an action that expects only
a single item to be selected.

We're using the GetDisabledReason field to enforce this (as well as DisabledReason
on menu items).

I've created a ListControllerTrait to store some shared convenience functions for this.

pkg/gui/controllers/commit_message_controller.go

@@ -14,11 +14,11 @@ type CommitMessageController struct {
 var _ types.IController = &CommitMessageController{}
 
 func NewCommitMessageController(
-	common *ControllerCommon,
+	c *ControllerCommon,
 ) *CommitMessageController {
 	return &CommitMessageController{
 		baseController: baseController{},
-		c:              common,
+		c:              c,
 	}
 }