Improvements to the "Process" extension.

- Make waitForFinished() return true also if the process is not running
at the time the function is called.
- Make exec() return -1 for a non-normal exit status;
QProcess::exitCode() is not guaranteed to return a non-zero value in
such a case.
- Introduce terminate() and kill() functions.

Change-Id: I130f6e31579c2e316a3534cd738deee848363a1d
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
Add support for C, Objective-C and Objective-C++ precompiled headers.

Task-number: QBS-278

Change-Id: I2999a64168f70aff21007e55d52314433425e84e
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>