Improve mapeditor tools
Improve selection tool. The user can now select features from a selection of layers
Fix selection tool. Disable the selection when the layer doesn't intersect the mapcontext extent
Merge irstv/master into improve_mc_bt
Merge pull request #726 from ebocher/improve_mc_bt

 Improve mapcontext butons #726 
Related to #722 and #721.
Fix #723 and improve the selection tool. The user is able to select geometries in several selected layers.