Adding tooltips to the interface?

[fedarko]

This is a small, low-priority thing I'd like to add in to Empress later on down the line -- recording it here for future reference.

We had some requests to add tooltips to the Qurro interface (screenshots here), and this didn't actually represent much added work -- once things are set up with Bootstrap, etc. it's only ~3 lines of HTML per tooltip element.

This could complement some of the more complex functionality @kwcantrell has been working really well -- for example explaining what we mean by Gradient, Trajectory, etc. in the context of animations.

[fedarko]

One thing worth mentioning: with #386, borders that match the background color are faster to add since nothing needs to be drawn in these cases. Would be nice to fit this into a tooltip for the barplot border options.