Initial input method support for the winit backend and the FemtoVG/Skia renderers

Known caveats:

- winit doesn't forward mouse events to the IME, so clicking
  with the mouse while composing results in funny effects such
  as the pre-edit text following the cursor.
- With FemtoVG there's no text decoration support, thus no underlining
  of the preedit area.

internal/compiler/builtins.slint

@@ -223,6 +223,7 @@ export TextInput := _ {
     property <bool> enabled: true;
     property <bool> single-line: true;
     property <bool> read-only: false;
+    property <string> preedit-text: native_output;
     //-default_size_binding:expands_to_parent_geometry
     //-accepts_focus
 }