Support clicking on barplots to show details

[fedarko]

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.

• 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.

[fedarko]

may be good to defer this until #318 is addressed, since there will likely be similar performance problems with this for large trees that'd benefit from some fancy math.

[fedarko]

This came up at this morning's meeting as another way (along with #366) to make barplots easier to explore.

It may be possible to make this somewhat faster than the "compute distance to all nodes" solution used for determining which node a user selected; since rectangular layout barplots are all at the same x-axis position (for a given layer anyway), and since circular layout barplots for a given layer are all at the same radius, we can just look at the y-coordinate (rect) or angle (circular: we can determine this for an arbitrary clicked position by computing arccos(x / radius)) and use that to figure out which tip to select.

I guess we can (when a click event happens) look at the position and check that it's past the first barplot layer? i.e. past the leftmost barplot layer's leftmost x-coordinate in the rect layout, or past the innermost barplot layer's innermost radius in the circular layout -- and use that to determine whether to pick the closest node or the closest barplot layer.

...hopefully shouldn't be too bad :)