MOBSDK-674: Final phase of WorkflowService refactoring

- Changed AlfrescoNode => AlfrescoDocument in attachment methods
- Introduced Filter to ListingContext and use it as a replacement for the retrieveProcessesInState and retrieveTasksForProcess:state methods
- Renamed public constants kAlfrescoTask... => kAlfrescoWorkflowTask...
- Added "Transition" to the Approve/Reject constant names
- Renamed "processDescription" property to "summary" on AlfrescoProcess and AlfrescoProcessDefinition classes
- Replaced the use of AlfrescoWorkflowVariable with the existing AlfrescoProperty class (also for consistency with Android)