io/key: [API] introduce FocusFilter for matching focus and editor events

Signed-off-by: Elias Naur <mail@eliasnaur.com>

io/key/key.go

@@ -128,6 +128,9 @@ type EditEvent struct {
 	Text  string
 }
 
+// FocusFilter matches [FocusEvent]s.
+type FocusFilter struct{}
+
 // InputHint changes the on-screen-keyboard type. That hints the
 // type of data that might be entered by the user.
 type InputHint uint8
@@ -357,6 +360,8 @@ func (SoftKeyboardCmd) ImplementsCommand() {}
 func (SelectionCmd) ImplementsCommand()    {}
 func (SnippetCmd) ImplementsCommand()      {}
 
+func (FocusFilter) ImplementsFilter() {}
+
 func (m Modifiers) String() string {
 	var strs []string
 	if m.Contain(ModCtrl) {