app,widget,io: implement IME positioning

This change implements reporting of the caret position from Editor, as well
as Windows, macOS, Android support. As a result, the IME composition window
on Windows and macOS is now positioned correctly.

References: https://todo.sr.ht/~eliasnaur/gio/246
Signed-off-by: Elias Naur <mail@eliasnaur.com>

internal/ops/ops.go

@@ -157,7 +157,7 @@ const (
 	TypeSemanticSelectedLen = 2
 	TypeSemanticDisabledLen = 2
 	TypeSnippetLen          = 1 + 4 + 4
-	TypeSelectionLen        = 1 + 4 + 4
+	TypeSelectionLen        = 1 + 2*4 + 2*4 + 4 + 4
 )
 
 func (op *ClipOp) Decode(data []byte) {