Change to use kb-bindings events instead of listening for keydown ourselves

Keybindings are now centrally handled using: https://github.com/deathcap/kb-bindings
This allows us to listen for the events using their (semantic) binding name,
and trigger only when the key is first pressed. Much cleaner since now only one
DOM keydown event listener is needed (in kb-bindings) and all the keybinding
definitions can be managed there (if managed by the end user, for example).

Requires https://github.com/deathcap/voxel-engine/commit/791c74ad818db9ed96475dd1e812debd709a8a1b
Add kb-binding-ui to let the user view and redefine keybindings if they want

https://github.com/deathcap/kb-bindings-ui
Merge master
Regenerate bundle.js