Move icon() from ExternalResolverGui to ExternalResolver to make ScriptCollection "GUI-less"
Move command factory from DatabaseCommand to Database
Use factories for creatig databasecommands in database

Thanks a lot to @rioderelfte for a long discussion on how to make this
more elegant - sorry, for not realizing your suggestions, too much work
for now just to be able to call commandname() statically.
Add API to access DBCommand factories so you can observe automatically created instances
Add Test for factory magic
Test a commandFactory is not available before the command was registered