1. Implemented "OpenFileCommand" recordable command.
2. Factored "DynamoModel.Open" method into "OpenInternal".
Merge branch 'ChocoButterUI' into OpenAndPlayCmds
Implemented "RunCancelCommand" recordable command
Updated both "Run" and "Cancel" buttons to use "RunCancelCommand"
Added TestRunCancelCommand and TestCreateNodesAndRunExpression test cases
Merge pull request #874 from Benglin/OpenAndPlayCmds

Accepting this since required test cases for other defects are pilling up (those that need to use the new RunCancelCommand).