Setting up a qbs profile, particularly for Qt, can take a measurable
amount of time, so we should not do bulk changes via the kitsChanged()
signal. Instead:
    - For the initial kit loading, delay the profile setup until
      the kit is needed for the first time, i.e. the user attaches it
      to a project.
    - Handle adding, removing and updating a kit separately; in
      particular, do not do anything to other kits.

Change-Id: If09d3c40b21e9a1356dc96089b92a60186a9bcdb
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>