Refactoring the seekbar widget to a new object.

So it can be reused in other parts of the program
Moving the select altitude Dialog to a new class. Using a seek bar to select the altitude.
Change the Altitude Dialog to use a numberPicker.
Moving drone pointer to the SuperActivity.
Using an interface for the altitudePicker callback.
Moving changeDefaultAlt() to the SuperActivity.
Using the AltitudeDialog to set guided mode altitude.

This Closes #77.
Merge pull request #78 from arthurbenemann/issue77

Issue #77
Better dialog for setting the default heigh.
Guided mode uses the default heigh.
Refactoring the seekbar Widget.