Debugger: Enable attaching with -client option.

Make it possible to trigger a debug-attach in a running instance
of Qt Creator by means of -client, use that in qtcdebugger, thus
enabling it to attaching to crashing executables in run mode
(Windows/CDB).

Modify IPlugin and application so that the complete command line is
serialized and transmitted to a running instance via QtSingleApplication
if -client is specified. Introduce IPlugin::remoteArgument and use that
in core for opening files and in the debugger for attaching.

Use -client in qtcdebugger with some logic to keep it alive as long as
the debuggee, make CDB break in that case as the events are not replayed
correctly in case the debugger is not spawned by the registered handler.
Rubber-stamped-by: con <qtc-committer@nokia.com>