Move V4 debugger agent into the debugger plugin

Move V4 debugger agent into the debugger plugin

The debugger is the only thing that actually needs it. Note that for
this to work we need to make QV4::Debugging::Debugger a QObject and
add some signals. The net effect is still a reduction in binary size
of about 1kb.

Change-Id: Ibecb8cfa140fc26fc13c8cbefb3d027ebdcd28a4
Reviewed-by: Erik Verbruggen <erik.verbruggen@theqtcompany.com>