Now that we remove binding sets, track binding set indices explicitly

Previously we relied on the length of the binding sets, but now that
we remove them the length is not a strictly increasing number. This
could cause problem when using the index to break specificity ties.

src/app/keymap.coffee

@@ -8,6 +8,7 @@ BindingSet = require 'binding-set'
 module.exports =
 class Keymap
   bindingSets: null
+  nextBindingSetIndex: 0
   bindingSetsByFirstKeystroke: null
   queuedKeystrokes: null
 
@@ -58,7 +59,7 @@ class Keymap
   bindKeys: (args...) ->
     name = args.shift() if args.length > 2
     [selector, bindings] = args
-    bindingSet = new BindingSet(selector, bindings, @bindingSets.length, name)
+    bindingSet = new BindingSet(selector, bindings, @nextBindingSetIndex++, name)
     @bindingSets.unshift(bindingSet)
     for keystrokes of bindingSet.commandsByKeystrokes
       keystroke = keystrokes.split(' ')[0] # only index by first keystroke