terrain mask work

adds an isValid(gridinfo tile) command to prefilter out grid elements of a specified nature
applies that to SetEmptyAction::process
also corrects sliderbar positions for terrain painting elements, as ell as adds a min/max slope for removal

potential TODOs: apply it more places pending feedback. add a min/max hieght filter as well

Engine/source/gui/worldEditor/terrainActions.h

@@ -54,7 +54,7 @@ class TerrainAction
          End,
          Process
       };
-
+      bool isValid(GridInfo tile);
       //
       virtual void process(Selection * sel, const Gui3DMouseEvent & event, bool selChanged, Type type) = 0;
       virtual bool useMouseBrush() { return(true); }