qml: hook any qml supporting plugins, add test plugin

This allows different platforms to have their own UI components while still leveraging the common
set of QObjects and utility components.

electrum/plugins/qml_test/__init__.py

@@ -0,0 +1,5 @@
+from electrum.i18n import _
+
+fullname = 'QML Plugin Test'
+description = '%s\n%s' % (_("Plugin to test QML integration from plugins."), _("Note: Used for development"))
+available_for = ['qml']

electrum/plugins/qml_test/qml.py

@@ -0,0 +1,12 @@
+import os
+from PyQt5.QtCore import QUrl
+from PyQt5.QtQml import QQmlApplicationEngine
+from electrum.plugin import hook, BasePlugin
+
+class Plugin(BasePlugin):
+    def __init__(self, parent, config, name):
+        BasePlugin.__init__(self, parent, config, name)
+
+    @hook
+    def init_qml(self, engine: QQmlApplicationEngine):
+        pass