Support clicking on barplots to show details

Analogously to how collapsed clades can be clicked on to show the clade's root node. Doing something similar here -- where clicked-on bars pop open a menu for their corresponding tip node -- would be really useful, esp. for datasets like the moving pictures tree where there are a few silly long branches that push out the barplots away from the rest of the tree.

Probably less important than #299, but would still be nice.

- [x] Pop up node selection menu when clicking on a bar
- [ ] Anchor node selection menu to the clicked-on position, rather than having it jump over to the position of the corresponding node
- [ ] Ensure that precision issues from before (where the wrong tip was selected when clicking close to the border of the bar) are resolved. I can't easily replicate these any more, but they may still be lurking in the code.
- [ ] Draw "selection" outline on the clicked-on bar? (not necessary.)
- This would be really useful if we can't immediately resolve the precision issues; showing the selected bar will make it clear if the selection was slightly "off" from what the user expected.