Create QML components only when the metadata source is different than the one used to create it.

The initial idea to achieve this:

1. create a hash from the (normalized) JSON data of each QML component.
2. Store the hashed values in a database.
3. Compare the values during the next import and generate QML components with unequal hash values.
4. Every time a QML component is edited, remove its hash.

This will only fly if the JSON created is stable. The JSON created by the plugin should be or can be made stable (By using stable ordered containers) but this JSON coming out of the design tool itself has to be checked.