Do not return as handled unless a geometry was modified

With this change, it is more straightforward to determine
whether an event is considered as handled, which results in the
removal of the modifiable_ and lastVertexCoordinate_ states.
Instead, we only need to know whether we're on a real vertex or
a virtual one. For that a new snappedToVertex_ flag is
introduced.

To stop a click after vertex deletion from triggering a feature
selection, vertex deletion is now triggered by a configurable
event condition, which defaults to singleclick (same as in the
select interaction) with no modifier keys.
Use the pixelTolerance to determine whether to snap to a vertex
Better documentation for the singleclick condition