Added new button to main window to synchronize stock with toppower
software
Added interface functions to ToppowerSync
Added id's to StockItem class (both in src and database)
Implemented StockManager <--> Toppower interface
Integrated ToppowerSync in the user interface.
The user can now synchronize the StockManager database with the Toppower
database (and vice versa)
Added support for article groups in Toppower
db changes
Merge remote branch 'origin/toppower_integration'

Conflicts:
	StockManagerGUI.py
	planten.s3db
	stockmanager.py
	
TODO: re-add planten.s3db database file.
Database is created when it does not exist.