[NUI] Add SystemFontSizeChangedManager class

[NUI] Add SystemFontSizeChangedManager class

SystemFontSizeChangedManager is a static class which adds user handlers
to the SystemSettings.FontSizeChanged event.

It is similar to SystemFontSizeChanged class but it is a static class
and it also provides Finished event handler invoked last after all
handlers added to the SystemSettings.FontSizeChanged event are invoked.