Cleaning
#41 Add snapping distance parameter to preferences dialog (but still not used in the code)
#41 Add a snapping distance parameter (currently not handled when changed in the interface).

TODO:
    * different parameters for grid and vertex snapping?
    * connect preferences dialog with snapping distance value
Fix SIGNAL / SLOT mistakes in connect
#41 Changing snapping distance in preferences is now handled (only for vertex snapping snapping)
#41 Different parameters for grid and vertex snapping distances
Use forward declarations
#41 Enable grid snapping parameter handling
Augment max value for spin boxes in preferences; Close #41