Dynamically add tip labels

[trvrb]

For SVG performance (especially when transitioning), we need to dynamically add and remove branchLabels and tipLabels from the DOM. With a zoomed tree with labels showing, I bet it's going to be safe to transition the ones that are visible.

[jameshadfield]

Somewhat temporarily fixed in c347f58 where I draw tip labels iff < 50 tips inView, and they appear after the branches/tips have transitioned

[jameshadfield]

• only active / "on" tips have labels
• new tips come into view via a L->R transition
• pan doesn't change active tips pan has been removed from the tree
• better tip in/out transitions
• the hardcoded # of branches selected should only count "visibile" tips
• changing tree layouts sometimes results in tips not being displayed when they should

[kairstenfay]

@jameshadfield should tip labels be re-calculated each time drawTips() is called?

Or rather, every time setLayout() is called?

[kairstenfay]

I believe commit 579c524 addresses the final bullet point.

[kairstenfay]

#728 addresses the parts of this issue where tip label visibility would not update after Date Range change, and where tip labels would not re-appear after layout change.

[jameshadfield]

Closing in favor of #915 #916