Refactor user_files module & add live reloading

- Cleaner design for the backend user_files classes and simplified
  interaction with QML
- Config and theme files will now automatically reload when changed on
  disk
- Removed manual reload keybind and button

src/gui/MainPane/TopBar.qml

@@ -40,12 +40,6 @@ Rectangle {
                         py.callCoro("get_theme_dir", [], Qt.openUrlExternally)
                 }
 
-                HMenuItem {
-                    icon.name: "reload-config-files"
-                    text: qsTr("Reload config & theme")
-                    onTriggered: mainUI.reloadSettings()
-                }
-
                 HMenuItem {
                     icon.name: "debug"
                     text: qsTr("Developer console")