Add support for detaching from a started process (like QProcess::detach())

Sometimes it would be really beneficial to be able to instantiate a QProcess, deal with it the way you want (setup channel modes, read/write, wait for something...) and then later on detach the process.

An example case in UI Extensions for Mobile project: http://qt.gitorious.org/uiemo/uiemo-preview/blobs/master/src/hbcore/theme/hbthemeclientqt_p.cpp#line69. HbThemeClient currently allocates QProcess on the heap without any parent just to avoid killing the process later on when the QProcess object would be destructed (a known memory leak there)!