factor out dedicated executable "marble-mobile" for mobile devices

Having a dedicated executable for mobile devices should ease maintainance for various reasons:
1) handling the smallscreen flag correctly is rather fragile and has lead to regressions in the past
2) allows for deeper integration into the system w/o "smart" code (determining network state, screensaver handling, hardware zoom buttons, etc.)

marble-qt looses the --smallscreen and --nosmallscreen commandline switches.

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