Linux: cefclient: Port to GTK3 (see issue #2969)

GTK3 is required by the Chrome runtime. The cefclient off-screen rendering
example no longer works with Ubuntu 16.04. With end-of-life in April 2021
we are dropping support for 16.04 in the near future in any case.

tests/cefclient/browser/browser_window_osr_gtk.h

@@ -117,8 +117,10 @@ class BrowserWindowOsrGtk : public BrowserWindow,
   static gint FocusEvent(GtkWidget* widget,
                          GdkEventFocus* event,
                          BrowserWindowOsrGtk* self);
+  static gboolean TouchEvent(GtkWidget* widget,
+                             GdkEventTouch* event,
+                             BrowserWindowOsrGtk* self);
 
-  void TouchEvent(CefXIDeviceEvent event);
   void RegisterTouch();
 
   bool IsOverPopupWidget(int x, int y) const;
@@ -176,7 +178,6 @@ class BrowserWindowOsrGtk : public BrowserWindow,
   static GdkFilterReturn EventFilter(GdkXEvent* gdk_xevent,
                                      GdkEvent* event,
                                      gpointer data);
-  static void InitializeXinput(XDisplay* xdisplay);
 
   XDisplay* xdisplay_;