New serialization plugin, allowing add multiple files/directories to the
single archive.
Added multipleFilesSerializationPlugin
Added FileTree and Button to dialogBuilder.
Merge remote-tracking branch 'origin/master' into multiple-file

Conflicts:
	src/main/java/meerkat/modules/core/App.java
Registered MultipleFilesSerializationPlugin
Fixed multiplefiles plugin preview
Merge pull request #1 from Ziemin/multiple-file2

Multiple file2