minor additions to database interface
Merge pull request #2 from solros/database_access

icon files and UTIs for polytope, graph and tropical
implemented limit and skip for db
changed input of additional properties to dictionary. Still not passed to db.
Merge remote-tracking branch 'upstream/database_access' into database_access
* now additional properties are sent to the database -- after entering a string of the form "CONE_DIM"=>5 in the textfield
* partly undoing the dictionary stuff
* tried to remove char*/std::string problem by moving the empty check into the perl function...