Issue #6: Support smaller displays such as the PiTFT

This commit adds support for smaller displays. It includes some resized graphics, a QVGA theme and the addition of two application arguments that can be used for testing PiPlug on various screen sizes. For example the following can be added to the "Program arguments" in the Eclipse launch configuration:

-width 320 -height 240

Task-Url: https://github.com/Genuitec/piplug/i/issues/issue/6