PYSIDE-164: Fix possible deadlock on signal connect/emit

Signal connect/emit acquire a lock on the QObject, and can
happen from python code (which has acquired the GIL) or
internal QtCode (without acquiring the GIL).
So we always need to release the GIL to prevent out-of-order
acquisition of the locks causing deadlock.

Change-Id: I1cf47a73c2b60627e322d8ef3fa4c3efdebd4c02
Reviewed-by: John Ehresman <jpe@wingware.com>