backend/sys: associate imported proxy with correct event queue

[cmeissl]

fixes a regression introduced in e777e85f6846ee732ab33adce5755bfc57b08649 which moved all proxies to a separate event queue but left proxies imported with Backend::manage_object on the default queue.

cc @chrisduerr

[chrisduerr]

I can confirm that this patch fixes the leak.

[chrisduerr]

Any reason to hold off on this? Seems better than the status quo.

Also wanted to link #747, so that gets closed when this is merged.

[chrisduerr]

Just for future reference: I won't be testing this in the future since I no longer use that in my browser. Or at last I soon won't be using it anymore. But it obviously still makes sense to fix.