ProjectExplorer: Clean up KitsManager API

Change-Id: Ib0b11918fbae9577928b6209a913afb3eccc48b5
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>
QbsProjectManager: Use qbs::Project as a value.

Those objects are shared data pointers that can (and should)
be used as values. This is now actually possible due to
the newly introduced public default constructor and
isValid() method.

Change-Id: I989e339bd495aa7581b01f17f00cbffe0b617b9c
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>