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Simplifies the code base by removing one level of unused
and unneeded abstraction.

- Merge {I,Default}AssistInterface to AssistInterface
- Merge {IAssist,Basic}ProposalItem to AssistProposalItem
- Merge {IGenericProposal,BasicProposalItemList}Model to GenericProposalModel

Change-Id: I54ee7b095427383d67a00fc1d87c3808c21d812d
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>