This yields a 10x paint performance increase at
#map=18.88/38.85208/-76.72632, as measured with
Chrome Canary's "Continuous Page Repainting" mode.
Fixes#885.
A keybinding now represents a set of key commands that can
be unbound as a set. Multiple keybindings are possible, and,
providing a namespace is provided to the constructor, will
not conflict with each other.
Also, key combination strings such as ⌘+A are now supported.
