Add highlight to multi-select sidebar on hover of entity

[jgscherber]

This is to resolve #2949 and is a WIP

This is my first time working on this project (and first time working in a large JS codebase) so please don't hold back on style/design criticisms. I want to thank you for the Architecture readme that you have. It really helped with initially finding my way around.

Here's a demo of the highlight

I ran into a couple issues while working on it

• After adding the hoverModeSelect callback, performing a click-and-drag results in iD dropping out of "select" mode. Any ideas what's causing that?
• The discussion in With multiple features selected, indicate which one is mouse-hovered in the sidebar multiselect list  #2949 says the resolution to this issue should also have the sidebar scroll into view the element in the list that's being highlighted - how can I check if an element is visible in the viewport?
• This change adds a class .hover to perform the styling which duplicates the pseudo-class :hover -- is it better for the :hover pseudo-class be removed and only styled using the .hover class

[quincylvania]

@jgscherber Thanks for working on this! I'll take a look and give you feedback this week.

[quincylvania]

No need to name this twice, you can pick either style.

[quincylvania]

This works fine but there's a .empty() function that's even easier.

[quincylvania]

@jgscherber Looks pretty good! We could merge as-is but I'll give you a chance to take another look if you want. Let me know if you have any questions.

After adding the hoverModeSelect callback, performing a click-and-drag results in iD dropping out of "select" mode. Any ideas what's causing that?

This was a bug with other changes I'd been making in the develop branch. If you remerge this should be fixed.

The discussion in #2949 says the resolution to this issue should also have the sidebar scroll into view the element in the list that's being highlighted - how can I check if an element is visible in the viewport?

I think we might actually live without this. I feel like it'd be disorienting more often than not. But to answer your question you can compare the list item element's position to scrollTop of the inspector.

This change adds a class .hover to perform the styling which duplicates the pseudo-class :hover -- is it better for the :hover pseudo-class be removed and only styled using the .hover class

Nah, there's no harm in two separate classes. I had to move all the :hover declarations to media queries anyway for #7432.

[jgscherber]

I'm planning on changing your two suggestions. I'll fix my rebase also