Added some persistent functionalities

- Added labelToCode attribute to VisibleProperty. Column names are only automatically set when this attribute is true.
If it is set to false, users can type the column name themselves
- Added tableName and labelToCode attributes to PersistentClass. Table name is automatically set if labelToCode is true.
If it is set to false, users can type the table name themselves
- Changed both mockup and class editor behaviour towards automatic name setting.
Merge branch 'master' of https://github.com/KROKIteam/KROKI-mockup-tool