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.
Now each platform-dependent module declares its interface in
platform/platform_module.h file and after that includes platform-
specific headers like platform/win/module_win.h with implementation.
Also removed the legacy WinRT platform-dependent implementations.
Ilya Fedin
John Preston