io/key: switch Event.Name to be a string and add function keys

Function keys don't have a natural rune representation so switch
Event.Name to be a string to fit "F1"-"F12".

Fixes gio#59

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

app/internal/window/os_ios.go

@@ -205,7 +205,7 @@ func (w *window) SetAnimating(anim bool) {
 	C.gio_setAnimating(w.view, animi)
 }
 
-func (w *window) onKeyCommand(name rune) {
+func (w *window) onKeyCommand(name string) {
 	w.w.Event(key.Event{
 		Name: name,
 	})