app: send keypress events for modifier keys

This change causes modifier keys (Control, Shift, Alt, Super) to be sent
to the application as key.Event events. These will still continue to be
used as modifiers for other key and pointer events as they are today.

This commit also adds a minor cleanup to use constants for function
keys in the OS-specific keypress handling functions.

Signed-off-by: Jeff Williams <kanobe@gmail.com>

app/os_macos.go

@@ -610,29 +610,29 @@ func convertKey(k rune) (string, bool) {
 	case C.NSPageDownFunctionKey:
 		n = key.NamePageDown
 	case C.NSF1FunctionKey:
-		n = "F1"
+		n = key.NameF1
 	case C.NSF2FunctionKey:
-		n = "F2"
+		n = key.NameF2
 	case C.NSF3FunctionKey:
-		n = "F3"
+		n = key.NameF3
 	case C.NSF4FunctionKey:
-		n = "F4"
+		n = key.NameF4
 	case C.NSF5FunctionKey:
-		n = "F5"
+		n = key.NameF5
 	case C.NSF6FunctionKey:
-		n = "F6"
+		n = key.NameF6
 	case C.NSF7FunctionKey:
-		n = "F7"
+		n = key.NameF7
 	case C.NSF8FunctionKey:
-		n = "F8"
+		n = key.NameF8
 	case C.NSF9FunctionKey:
-		n = "F9"
+		n = key.NameF9
 	case C.NSF10FunctionKey:
-		n = "F10"
+		n = key.NameF10
 	case C.NSF11FunctionKey:
-		n = "F11"
+		n = key.NameF11
 	case C.NSF12FunctionKey:
-		n = "F12"
+		n = key.NameF12
 	case 0x09, 0x19:
 		n = key.NameTab
 	case 0x20: