Spliting of a Latex file into multible files is now possible.

# Splitting of Latex files into Master and sub documents is possible.

I have changed flylatex such that it can handle split Latex files into several documents. E.g a master document and several sub documents.

## Changes
### DB Changes:
To enable this an additional db layer was implemented called Project. This is a handler for several documents. Where one of the documents is the master document.
The DocPrivileges was changed to contain one project instead of one document.

### Sharing:
For sharing I decided that it does not make sense to share individual documents. Only whole projects are synced between the users.

If an user adds an subdocument to an project, the UI of the other users is automatically updated.

### User Interface:
I added a dialog to add sub Documents to one of the existing master documents.


## Limitations

 * So far only Latex documents can be splitted.
 * No sub folders are possible

## Feedback
Hope the code matches your quality criteria. If you have any questions I am willing to help you. Also if you are not happy with some design decisions please inform me such that I can change them.