make the android => qbs dependency optional

as qbs itself is optional

Change-Id: I1ff2d2f785ca206b379b305e783b9914081712b4
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
Qbs project files: Use groups wherever applicable.

Provides better structuring of source files.

Change-Id: Ic2d0094312bb7c8da01a1f38270564407fdbefce
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>