feat: allow setting window shape (backport: 3-0-x) (#14445)

* feat: allow setting window shape

This binds Widget::SetShape, an API that already exists in Chromium (for
Windows and Linux). It's a more reliable method of having some parts of
your window be "click-through" than the current `setIgnoreMouseEvents`
API, which messes around with the `WS_EX_LAYERED` window style on
Windows, causing strange bugs and incompatibility with hardware
acceleration.

* update docs

atom/browser/api/atom_api_top_level_window.h

@@ -146,6 +146,7 @@ class TopLevelWindow : public mate::TrackableObject<TopLevelWindow>,
   bool HasShadow();
   void SetOpacity(const double opacity);
   double GetOpacity();
+  void SetShape(const std::vector<gfx::Rect>& rects);
   void SetRepresentedFilename(const std::string& filename);
   std::string GetRepresentedFilename();
   void SetDocumentEdited(bool edited);