Move diff_match_patch stuff to a new 'thirdparty' directory.

Since many components will use the diff_match_patch library (namely, the filemanager, the collaborative and the diff components at least), I propose that we move it to a new 'thirdparty' directory.
In the future, this directory could contain also a git submodule with the current ace build, and potentially the gcli command line code that we might use in a 'commandline' component. This would allow us to update them easily when new releases come from upstream.
I wanted to wait a bit for this pull request but since I see movement towards a 'diff' component I propose it now.
I know that creating a 'thirdparty' directory has not been discussed yet in an issue, so I am totally open for discussion.