Solved MOSYNC-2818 and MOSYNC-2829

MOSYNC-2818: The back button is handled inside the screen class - it should be handled separately by every screen type (TabScreen, StackScreen and Screen).
Also, an event needs to be sent to mosync when the back button is touched.

MOSYNC-2829: On wp7 platform 'maGetScrSize()' returns the same values disregarding the screen orientation.