qt: fix "Cannot move to target thread" warning getting spammed with Qt 6.9.2

We can use blocking queued connection, because the plugins are instantiated once and waiting can be tolerated.

This fixes the following warning getting spammed with Qt 6.9.2:

QObject::moveToThread: Current thread (0x7f6c48000f00) is not
the object's thread (0x7f6c140021d0).
Cannot move to target thread (0x7f6c48000f00)