This commit adds a new avoid intersection mode setting when
digitizing new features. The three available modes are:
- allow intersections/overlaps
- avoid intersections/overlaps on active layer
- avoid intersectonss/overlaps on layers list
The third mode is what QGIS has had for a while, except
its UI/UX is a bit messy. The layers list is setup by
the user via the advanced snapping configuration widgets,
but isn't connected at all with whether snapping is
enabled / disabled.
This new approach makes it explicit to user whether
newly-added features might be clipped or not, and
if so by which layer(s).
Finally, the new 'avoid intersections/overlaps on
active layer' is likely a far more useful behavior
than having a list of layers (for e.g., you might
be digitizing on a layer that can't have overlap
with itself but is fine to overlap with another
layer, the latter also in need of avoid overlap
_with itself_).
