Specifying keybindings that use a command key (⌘ on Mac,
Ctrl on Win/Linux) using the appropriate Mac binding, e.g.
'⌘Z'. Use iD.ui.cmd to translate it to an appropriate key
string for other platforms, e.g. 'Ctrl+Z'.
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.
