Move RecordableCommands to DynamoCore

The Dynamo should be able to execute RecordableCommands in headless mode (when there is no WPF form). Thats why they should be in nonUI part of core - DynamoCore. Also their Execute method implementation should be based on DynamoModel rather than DynamoViewModel, so RecordableCommands are UI independent

This PR contains a lot of changes but most of them are minor: DynCmd.SomeCommand -> SomeCommand, DynamoViewModel.SomeCommand -> SomeCommand
Well... So this is an interesting question. It does need to be in Flood in the longer term as we can use it to test Flood. Or we could write another target that talked only to the wire protocol. 

The only thing that it really needs other than manipulating the model is to be able to participate in the undo/redo stack. I'm a little surprised that you say it needs a lot of non-trivial changes to split. Could you give me an example of why that's the case?