Move getElementFromSelector & getSelectorFromElement

[GeoSot]

Move getElementFromSelector & getSelectorFromElement inside selector-engine.js, in order to use SelectorEngine methods, avoiding raw querySelector usage

Feature: add getMultipleElementsFromSelector helper

[julien-deramond]

PR LGTM but I got a question of architecture.

For the Manipulator we can do stuff like Manipulator.getDataAttributes(). We don't want here to be able to do SelectorEngine.getElementFromSelector(), SelectorEngine.getMultipleElementsFromSelector() and SelectorEngine.getSelectorFromElement()?
It would probably allow to export it like export default SelectorEngine and so not being forced to import it with brackets like we had to do for example in import { SelectorEngine } from '../../../src/dom/selector-engine'. And would be consistent with all the files in js/src/dom/*.js that are built like this.

Was thinking of something like that: patch.txt (git apply patch.txt to see the diff)

[GeoSot]

Of course, I am ok with this approach. I tried to do the minimum changes.
I have applied your suggestion on e96546d

[julien-deramond]

This change LGTM! I let others double-check that :)