Fixed duplicate display of dashboard after car...

... selection.
Cache cars to be able to select cars without...

... internet connection.
Prevent app from trying to download tracks...

... if no internet connection is available.
Enable creation and use of new car if no...

... internet connection is available.
Assign temporary id to new cars..

... that can at time not be uploaded to the server. Temp ID allow the
creation of LOCAL tracks with different LOCAL cars. The local cars will
be uploaded to the server before the track is uploaded.
This fixes #130 and #166 plus some minor issues