QtcProcess: Implement waitFor...() methods in general way

In this way remove ProcessInterface::waitFor...() methods, as the general implementation should work for all ProcessInterface subclasses.

The idea is that when calling certain QtcProcess::waitFor...() method, the QtcProcess moves temporarily the internal ProcessInterface object into a new thread and use a blocking QWaitCondition for communication with the ProcessInterface. Afterwards the temporary thread moves the ProcessInterface back to the main thread. The communication should look similar to the one implemented between CallerHandle and LauncherHandle.