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[FEATURE] Tracing of features (digitizing) Tracing can be now used in various capturing map tools (add feature, add part, ...) including reshape and split tools. Tracing is simply a new mode for these tools - when tracing is not enabled, the tools work as usual. When tracing is enabled (by clicking the new magnet icon or pressing T key), tools switch to tracing behavior: - first click on a vertex/edge (must be snapped!) will start tracing - moving mouse on top of the map continuously updates the trace - next click will confirm the trace and mark start of a new trace Tracing can be enabled/disabled anytime even while digitizing one feature, so it is possible to digitize some parts of the feature with tracing enabled and other parts with tracing disabled. Tracing respects snapping configuration for the list of traceable layers. If there are too many features in map display, tracing is disabled to avoid potentially long tracing structure preparation and large memory overhead. After zooming in or disabling some layers the tracing is enabled again. Internally, things work like this: - when tracing is requested, linestrings are extracted from vector layers, then noded (using GEOSNode to resolve all intersections) and finally a simple planar graph is built (vertices + edges) - when tracing, endpoints are temporarily added to the graph (if not equal to one of existing vertices already) and Dijkstra's algorithm is run to get shortest path Original specs for the curious ones (the interaction with QGIS is slightly improved from what has been specified): http://www.lutraconsulting.co.uk/crowdfunding/autotrace-phase-2/specification.pdf