TagManager:
 - db method implementation
 - all actions in the manger call appropriate db methods
The Main Tags table is no longer erased every time COW is re-started
Use the id instead of the name in the creation, modification and deletion of main tags in the tag manager
Add Favourite Container (TODO: store favourites in the DB, currently they are lost when the user logs out)
Add Custom Container (TODO: I cannot store more than one object in an array as an attribute...don't know why
A GlobalContainer is created in the global space for every Main Tag (TODO:  Fix the x,y coordinates at creation time )
Add placeholder-text for the global container
Make Global Containers droppable, now the main Tag is changed when drag and drop file representations between the containers
Fixed an error with missing image previews (now using the temp folder of the operating system)
when a  GlobalContainer is created than it is placed in the global space
simple small bugfix
Add Database Connection for user favourites (TODO: DB not reliable, deletion and insertion of new entries works only in 2 of 3 cases. After an error, the favourites of this user are lost)
Solve the TODO for the Custom Containers, now more than one file can be stored
Change the ObjectManager.createobject method to improve the reliability of object creation
Small change in the  ObjectManager.createobject method
Change the storage of searching and sorting preferences from attributes to JSON-Objects to ensure that everyone can work simultaneously with the same containers
Now the Global Container only show these file objects which have a mainTag which corresponds to the name of the Global Container
another small change in the  ObjectManager.createobject method (I hope to be the last one)
FavouriteContainer: Move the add and remove functions to the client side (unfortunately the problem when storing more than a few elements in the DB remains...)
Solve the TODO for the favourite Containers (the DB simply needs more time to remove elements)