Fix calls build in Xcode. Fix calls panel in Retina.

Also implement panels that appear in all spaces on macOS.
Using them for calls panels and custom notifications, so it
will be possible to use custom notifications in macOS as well.

Telegram/SourceFiles/platform/linux/specific_linux.cpp

@@ -405,6 +405,9 @@ bool TransparentWindowsSupported(QPoint globalPosition) {
 	return false;
 }
 
+void InitOnTopPanel(QWidget *panel) {
+}
+
 namespace ThirdParty {
 
 void start() {