ProjectExplorer: Remove project parameter from PEP::canRun()

.. and rename it to canRunStartupProject().

It's only ever used with the startup project. No need to let
all callers retrieve it by themselves.

Change-Id: I24e56ce339bde70d96e28db51e6ddcf75c277ceb
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>
Reviewed-by: Tobias Hunger <tobias.hunger@theqtcompany.com>