New feature info interface

[dasmoth]

Feature info popups are the only place where the old popup system is used. They don't work well on touch-screen devices, and have severe formatting problems when large amounts of information is attached to one feature.

They should be replaced by a non-popup system, probably using the same sidebar as the track adder and editor.

Things to consider

• Existing FeatureInfoPlugins should Just Work.
• Would be good to have finer-grain control of which "default" fields appear -- probably by moving these to their own FIPs.
• Can we do a better job of representing what's going on with complex features?