[5SP] UI: Port the rest of OD to messaging-based graph abstraction

[ivan-cukic]

#200 introduced abstract graph model that communicates with GR only via messages. There are remaining parts of the UI that need to be ported:

• GR is missing messages for available block types #247 (@drslebedev )
• Replace data in the block creation dialogue with the data from the previous item
• Remote sources are added using the old implementation, replace with message-based creation
• Initial block layout needs to be implemented (topological sort-based layouting is implemented, but not automatically performed yet)
• Initial graph loading in OD is based on the old approach. Reimplement initial loading not to use old Block and Connection classes
• Currently, OD sends messages to GR via a singleton App which needs to be changed as we can have more graphs to edit at the same time (nested graphs / hier-graphs, remote graphs)
• Types that are communicated to the user are mangled, as reported by GR
• Restore port colours (previous item needed)
• Block navigation arrows need to be reimplemented replaced by [3SP] UI: Redesign the graph traversal in the flowgraph view #202
• Reimplement visual block filtering
• Code needs to be cleaned up (delete all the code from the old non-message based parts of the UI - notably the old Block and Connection classes)