fixes https://github.com/spesmilo/electrum/issues/7750
Each task we schedule on `TaskThread` can provide an optional `cancel` method.
When stopping `TaskThread`, we call this `cancel` method on all tasks in the queue.
If the currently running task does not implement `cancel`, `TaskThread.stop` will block
until that task finishes.
Note that there is a significant change in behaviour here:
`ElectrumWindow.run_coroutine_from_thread` and `ElectrumWindow.pay_lightning_invoice`
previously serialised the execution of their coroutines via wallet.thread.
This is no longer the case: they will now schedule coroutines immediately.
So for example, the GUI now allows trying to pay multiple LN invoices "concurrently".
96c063028a