Allow flow specification ends to reach down into feature groups

[lwrage]

Flow specifications at leaf components should be allowed to reach arbitrarily deep into feature groups. Such flow specifications can only have trivial flow implementations (if any).

• Update meta model
• Update grammar
• Update parsing, name resolution, content assist
• Update formatter (how to get rid of spaces in dotted name?)
• Update validator (existing models)
• Validate: no reach down in flow impl, no flow impl for spec with reach down
• Update flow instantiation
• Create tests
• Update graphical editor (similar to connections?)

This implements saeaadl/aadlv2.2#29