Separate between overridden values and others in command-line frontend.

The current code is too simplistic in that it just pretends the whole
configuration was explicitly set on the command line. This can lead to
precedence problems regarding module properties set in project files and
profiles.

Change-Id: I50113486159308a9660be7b86d2e2240527df73e
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
Rename "qbsPath" preference setting to "qbsSearchPaths".

The old name is grammatically incorrect and also not descriptive enough.
The new one is also consistent with the project and product
properties that amend the respective setting.

Change-Id: I7be6b3d986fdb1f232623bf9b471ab21e04ed9f2
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>