[UX Review] Awkward mouse down and hover interaction

[ellisonbg]

This is going to be hard to describe in screenshots, but I will try.

First hover...when you move your mouse cursor up and down over the dataset browser, a vertical grey rectangle follows the row you are on, but there are solid grey rectangles on all parents:

For the hover state, I recommend that we only show a hover indicator on the dataset row that the mouse is currently over, and omit the hover indicators on the parents. Also, once we implement the layout of #121, I think it makes sense to use a hover indicator on the entire row (similar to the JLab file browser).

For the mouse down state, those grey hover/selections rects turn a darker shade of grey. Having a mouse down interaction is awkward because of how it interacts with selection. I recommend removing the mouse down handler.