Add toggle to automatically connect new vertex to edge underneath it

[jvdwetering]

Adds a button next to the Selection/Vertex/Edge buttons which when toggled makes the 'add vertex' automatically connect on an edge underneath it.

To make clear it snapped I added an animation to the edge, for which I added a new EItemAnimation class.

[jvdwetering]

This is related to #311, but then kind of the converse. It is essentially the same feature as the 'magic slice' identity addition, but then using the tools native to the edit window.

[RazinShaikh]

This is very hit or miss. Can you do something about that?

snap-to-edge.mp4

[jvdwetering]

I've mostly been dealing with this by zooming in more, so that the edge is wider. An alternative solution I was thinking of, was to make an edge you are hovering over more thick so that it is easier to click on it.
There is also the manual solution, where we don't use the built-in Qt collision features, but instead check ourselves across all edges whether there is an intersection (or something close to it), this might get slow for large diagrams though.
I should also add that the current implementation suffers from the same issue described in #302.

[jvdwetering]

This now also implements #311.
I think this is ready for merging

[RazinShaikh]

looks good to me

[jvdwetering]

This is giving mypy a hard time:

def it(self) -> EItem:
        if self._it is None and self.scene is not None and self.e is not None:
            self._it = self.scene.edge_map[self.e]
        assert self._it is not None
        return self._it

we have _it: Optional[EItem].
The error is Incompatible types in assignment (expression has type "Dict[int, EItem]", variable has type "Optional[EItem]").
I don't understand why it is an error to assign a EItem to something of type Optional[EItem].

[RazinShaikh]

Hi @jvdwetering , thanks for merging this. For the next time, do you mind merging the PRs without squashing the commits. I think for a big PR like this, it is quite useful to have the full commit history for future reference/debugging instead of a single commit containing all the changes.

[jvdwetering]

Okay that makes sense. I wasn't sure what a good policy would be.