Fixed two selection/picking related bugs

1. RTreeNode bounds calculation now always uses the detailed geometry of
   district edge nodes to calculate their bounds. This fixes picking to
   work in cases where the detailed geometry falls much outside of the
   spanning rectangle of the edge's end vertices.
2. Introduced DistrictSelectionNode that tells G2DParentNode to ignore
   its bounds instead of nullifying them entirely. This caused the edge
   nodes to fall outside the R-tree as "boundless nodes" which caused
   the most optimized R-tree based picking to fail entirely for nodes
   that are selected when RTreeNode.decompose is executed.

gitlab #68
gitlab #69

Change-Id: I119f33a04923b9b4bf06a8229cfccedc520588cb