Added functionality to start, stop a client process plugin

Added functionality in the PluginManager to start and stop process
plugins. Did not want to modify PluginManager to a QObject unless and
until required. So made use of the waitForStarted() and
waitForFinished() methods to block the process startup, shutdown.

Signed-off-by: Sateesh Kavuri <sateesh.kavuri@gmail.com>