add import dialog for parsed cna models

[rempel]

import wizard for cna models to allow changing of slot-names and compound-type (some-how/and/not are completely determined from data in cna file)

1. change data structure (metabolite) to include compound-type and slot-name
2. refactor import code to support creation with different names + types

start with identity map

3. wizard between parsing and slot creation step, to change slot-names + compound-type necessary to use sensible default values
4. change creation code to use values from name-map, type-map;

[rempel]

class change is possible via context menu for each slot, refactoring done, but not thoroughly tested. dialog for type assignment indefinitely postponed.