[Qt] Improved ElectrumWindow (wallet main window) close-window cleanup
[Qt] Improved ElectrumWindow (wallet main window) close-window cleanup

- Unregisters self from all network callbacks on window close (previously it
didn't completely unregister)

- Disconnects all of its instance signals named "*_signal", so as to
avoid spurious events arriving to a not-yet-GC'd wallet window

Note: The ElectrumWindow still isn't getting GC'd.  I suspect bound
lambda connections or other weirdness is causing the instance to stick
around.  TODO: Fix this situation

fyookball/electrum,fyookball/electrum,fyookball/electrum