allow enable/disable proxy without nuking proxy mode, host and port (explicit enable_proxy config setting),
move tor probe from frontend to backend code, add probe buttons for Qt and QML
```
=============================== warnings summary ===============================
tests/test_wizard.py::ServerConnectWizardTestCase::test_no_advanced
tests/test_wizard.py::ServerConnectWizardTestCase::test_proxy
tests/test_wizard.py::ServerConnectWizardTestCase::test_proxy_and_server
tests/test_wizard.py::ServerConnectWizardTestCase::test_server
tests/test_wizard.py::WalletWizardTestCase::test_2fa
tests/test_wizard.py::WalletWizardTestCase::test_create_standard_wallet_haveseed_bip39
tests/test_wizard.py::WalletWizardTestCase::test_create_standard_wallet_haveseed_electrum
tests/test_wizard.py::WalletWizardTestCase::test_create_standard_wallet_newseed
/tmp/cirrus-ci-build/.tox/py3/lib/python3.10/site-packages/_pytest/threadexception.py:82: PytestUnhandledThreadExceptionWarning: Exception in thread Plugins
Traceback (most recent call last):
File "/usr/local/lib/python3.10/threading.py", line 1016, in _bootstrap_inner
self.run()
File "/tmp/cirrus-ci-build/electrum/plugin.py", line 360, in run
self.on_stop()
File "/tmp/cirrus-ci-build/electrum/util.py", line 430, in on_stop
loop = get_asyncio_loop()
File "/tmp/cirrus-ci-build/electrum/util.py", line 1578, in get_asyncio_loop
raise Exception("event loop not created yet")
Exception: event loop not created yet
warnings.warn(pytest.PytestUnhandledThreadExceptionWarning(msg))
```
