Debugger: Do not enable QML-debugging by default for projects using C++.

QML-debugging is rarely desired for C++ projects and causes warnings
about timeouts from the QML debugger not being able to connect.

Change-Id: Ide3e870a40b49deae24d3d88ea4973dd45d83e75
Reviewed-by: hjk <hjk121@nokiamail.com>
Improve performance of writing settings

By wrapping related ones in a single transaction, instead of
(implicitly) doing a transaction per settings value that is written.

Task-number: QTCREATORBUG-11524
Change-Id: I28e06c9d6cd4ebad024f9c277796abb37df5f992
Reviewed-by: hjk <hjk121@nokiamail.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>