GenericProjectManager: General editor related code consolidation

Convert to new editor setup scheme, minor cleanups.

Change-Id: I1ac9169b9d397500317595a1a964347b0f7de19d
Reviewed-by: Christian Stenger <christian.stenger@digia.com>
QbsProjectManager: Make DefaultPropertyProvider a bit more readable.

- Decrease indentation by exiting early.
- Use switch instead of "else if".

Change-Id: I74e73e087f64c16f01497854c67f581ef79aff3c
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>