[draft] Add node/link selection feature

[mwarning]

Select a node or link while holding the CTRL key. This code works but needs proper integration and better highlight colors for the map view. It is meant to serve as an inspiration on how to implement this feature.

A bug was also fixed: The on 'click' event handler may only be installed once, otherwise it will called for every time the renderer is initialized (when switching between graph and map).

Checklist

• [x] I have read the OpenWISP Contributing Guidelines.
• [x] I have manually tested the changes proposed in this pull request.
• [ ] I have written new test cases for new code and/or updated existing tests for changes to existing code.
• [ ] I have updated the documentation.

Reference to Existing Issue

Closes #.

Please open a new issue if there isn't an existing issue yet.

Description of Changes

Please describe these changes.

Screenshot

out.webm

[nemesifier]

@mwarning is this directed at #163?

[mwarning]

@nemesifier yes, it is directed towards that ticket.