QbsProjectManager: Fix command-line representations.

- All except the one for "qbs build" were incomplete, which is fixed
  now.
- The new implementation is centralized, so maintenance will be simpler.
- The command lines are also completely self-contained now, so they can
  be pasted as-is into a shell with no assumptions about the working
  directory etc.

Change-Id: I2c684141bcdc5c6da0e1af60ce60278fc4dcd088
Reviewed-by: Jake Petroules <jake.petroules@petroules.com>