Analyzer: Replace StartMode with a useStartupProject boolean

This is what it is on the top level. The change makes it obvious
that in the valgrind(-derived) plugins the value is later wrongly
used to make a decision on whether to run the valgrind process
locally or remotely. But that's isolated in valgrind now and
can be fixed there.

Change-Id: I6fa5e669dec1f9e2cdebe42a1591d15144082a21
Reviewed-by: Anton Kreuzkamp <anton.kreuzkamp@kdab.com>
Reviewed-by: Christian Stenger <christian.stenger@theqtcompany.com>
Reviewed-by: Ulf Hermann <ulf.hermann@theqtcompany.com>