Add support for select-all(), cut(), copy() and paste() functions on text input elements (#2804)

In the compiler this is still very primitive, but an attempt to start a
generic interface. The basic assumption is that all item functions will
eventually need access to the window adapter and itemrc. Support for
additional arguments is still missing.

Also missing is support for the function access via rtti in the
interpreter, hence the hardcoding at the moment.

api/cpp/include/slint.h

@@ -265,6 +265,9 @@ public:
         }
     }
 
+    /// \private
+    const cbindgen_private::WindowAdapterRcOpaque &handle() { return inner; }
+
 private:
     friend class slint::experimental::platform::SkiaRenderer;
     friend class slint::experimental::platform::SoftwareRenderer;