Initial commit for real hidpi support

Note: This commit has only been tested on MacOS

Changes:
- Icons now use the native hidpi support from Gtk (through Icon Theme)
- Icons are now directly generated from scalable file (i.e. SVG file)
- Widget sizes are scaled based on DPI and scale factor
- Font size is scaled based on DPI and scale factor

rtgui/cursormanager.h

@@ -81,7 +81,6 @@ private:
 
 public:
     void init                         (Glib::RefPtr<Gdk::Window> mainWindow);
-    void cleanup                      ();
     static void setWidgetCursor       (Glib::RefPtr<Gdk::Window> window, CursorShape shape);
     static void setCursorOfMainWindow (Glib::RefPtr<Gdk::Window> window, CursorShape shape);
 };