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.
Sometimes NSUserNotificationCenter -deliveredNotifications method call
freezes for a long time, so now we use it only in a separate thread and
we group all the requests for clearing while another clearing is done.
Don't deactivate the application when the main window is hidden.
Such behavior provides some unwanted windows reordering in the
current workspace when the window is hidden by Cmd+W.
Ignore app activation by applicationDidBecomeActive: notification
for a short period of time after a user notification for other app
instance was received (the system sends them sometimes and the main
window is shown + activated for a wrong instance of the application).
Currently the build without implicitly included precompiled header
is not supported anyway (because Qt MOC source files do not include
stdafx.h, they include plain headers).
So when we decide to support building without implicitly included
precompiled headers we'll have to fix all the headers anyway.
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.
