Cards & Settings refactoring

- All cards now live in Modules/Cards
- CalendarPanel is now called ClockPanel
- Added a way to ease settings migration in separate QML files

Commons/Migrations/MigrationRegistry.qml

@@ -0,0 +1,15 @@
+pragma Singleton
+
+import QtQuick
+
+QtObject {
+  id: root
+
+  // Map of version number to migration component
+  readonly property var migrations: ({
+                                       26: migration26Component
+                                     })
+
+  // Migration components
+  property Component migration26Component: Migration26 {}
+}