[EAK-396] Implement merge mode when storing autogenerated content into a package

[smiakchilo]

EToolbox Authoring Kit plugin operates in two modes regarding the content of a package:

1. WriteMode.OPEN: searches for the folder of a component; if found, removes all the .XML files from it and instead stores the autogenerated xml-s, such as _cq_dialog.xml. If folder is not found, an exception is thrown (default);
2. WriteMode.CREATE: searches for the folder of a component; if found, does the same as (1); otherwise, creates the folder.

Per users' request, we need the merge mode. This one

• will work generally the same as (1);
• but the XML-s found already in the package won't be wiped;
• if an XML file is found with the same name as the file that the Plugin intends to write (e.g., _cq_dialog.xml):
  • this file is read into memory;
  • the autogenerated file is stored;
  • the previous file is merged onto the autogenerated one, node-per-node and attribute per attribute (this will help to implement a "monkey patch" of the autogenerated XML if user is lacking some feature)