Add CLI arguments parsing + --start-in-tray

- Loading a QML file in place of src/gui/UI.qml is now possible via
  -l / --load-qml instead of taking the first positional argument

- New option to start application in tray without showing a window

- --help and --version

src/gui/qmldir

@@ -1 +1,2 @@
 singleton ModelStore 0.1 ModelStore.qml
+singleton ArgumentParser 0.1 ArgumentParser.qml