WMS provider fixes for multi-threading

This is still far from working nicely or being finished:
- cloned provider does not get capabilities copied over, so they are requested again (for each redraw!)
- legend graphic download is disabled for now, was causing crashes while resizing window / manipulating docks
- tiled requests probably do not work
WFS: added featureSource() - now it is possible to do rendering
WCS: quick and dirty way to get it working
GRASS raster: remove superfluous QProcess instances
GRASS provider updated to support featureSource()
GPX: added support for featureSource()
delimitedtext: move methods specific to feature iterator from provider to FI
Fix issue with combination of edit buffer + vector join