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
 }

internal/compiler/widgets/common/common.slint

@@ -40,7 +40,7 @@ export LineEditInner := Rectangle {
     placeholder := Text {
         height: 100%; width: 100%;
         vertical-alignment: center;
-        text: root.text == "" ? root.placeholder-text : "";
+        text: (root.text == "" && input.preedit-text == "") ? root.placeholder-text : "";
     }
 }