Add graph "trailing mode" to keep recent active symbols in graph

[egraether]

Currently when navigating from symbol to symbol in the graph you don't see where you came from/where you have been.

Add a button for 'trailing mode' at top of graph view. When activated it keeps the last active symbols if connected to with each other and the current active symbol.

The trail is removed from the graph when:

• a new search/text search is done
• overview is shown
• errors are shown
• unconnected symbol is activated via code view
• file is activated