Manage generator rules for an M0 project

[schubmat]

For a given M1 project generation rules can be defined. Either they have been inherited from the preceding version, or the have to be configured from scratch. In any case,

• the M1 project has a default generation which is used for any new M0 project
• in case no generation is specified, the generation of M0 artifacts is not possible.
• in case multiple generation rules are present, and the user does not want to use the default selection, the user needs to choose which M0 generation of the given M1 model will be used
• if there is none yet, a user can click "define export rules" (see ticket #TODO)