Set groundwork for better disabled reasons with range select

Something dumb that we're currently doing is expecting list items
to define an ID method which returns a string. We use that when copying
items to clipboard with ctrl+o and when getting a ref name for diffing.

This commit gets us a little deeper into that hole by explicitly requiring
list items to implement that method so that we can easily use the new
helper functions in list_controller_trait.go.

In future we need to just remove the whole ID thing entirely but I'm too
lazy to do that right now.

pkg/gui/types/common.go

@@ -242,6 +242,12 @@ type MenuItem struct {
 	Section *MenuSection
 }
 
+// Defining this for the sake of conforming to the HasID interface, which is used
+// in list contexts.
+func (self *MenuItem) ID() string {
+	return self.Label
+}
+
 type Model struct {
 	CommitFiles  []*models.CommitFile
 	Files        []*models.File

pkg/gui/types/context.go

@@ -136,6 +136,7 @@ type IListContext interface {
 	Context
 
 	GetSelectedItemId() string
+	GetSelectedItemIds() ([]string, int, int)
 	IsItemVisible(item HasUrn) bool
 
 	GetList() IList

pkg/gui/types/suggestion.go

@@ -6,3 +6,8 @@ type Suggestion struct {
 	// label is what is actually displayed so it can e.g. contain color
 	Label string
 }
+
+// Conforming to the HasID interface, which is needed for list contexts
+func (self *Suggestion) ID() string {
+	return self.Value
+}