Add a feature to display notification when user goes to homescreen

[ksiazkowicz]

Show a global query when user goes to homescreen and has unread messages. After reading the notification, don't show it again until user gets a new message.

Useful docs: http://developer.nokia.com/Community/Discussion/showthread.php/228048-How-to-check-if-HomeScreen-is-in-foreground

It should work like built-in messaging app's notification seen also in Nokia Chat notification, WhatsApp and Palringo.