Feature filtering

This addresses #1299, #1871, #2033 

Adds:
* flexible feature definition
* sidebar to allow user to toggle features (hotkey 'f')
* info bar on bottom to warn user when features are hidden
* supports auto-hiding features (currently auto-hides >100 points, >100 buildings)

Not ready to merge yet, but looking for feedback.
I really like how this is coming together - dense areas are way easier to edit now.

TODO:
* replace hamburger icon
* adjust text/tooltip language?
* better handling of multipolygons? (e.g. should really hide inner 'holes' when hiding an 'outer' landuse)
* restrict some edits if a feature is hidden? (e.g. moves, deletes?)
* add wireframe mode? (#813)
* make gpx tracks features instead of background? (related #2352, #2335)

![screenshot 2014-09-16 16 02 48](https://cloud.githubusercontent.com/assets/38784/4294306/5bbdca92-3dde-11e4-9faa-339e99e47acc.png)