Make Wayfire's default bindings optional

[ammen99]

With #1501, bindings will stop being a special case in Wayfire's input handling system and can be implemented as a plugin. Therefore, it would make sense to move bindings to a separate plugin which provides an API for registering bindings on an output. This API should make it possible for an alternative implementation of a plugin for handling bindings, which can then for example support multi-stroke bindings, or bindings based on the xkb state instead of raw keycodes, etc. transparently to existing plugins.

As this will be a very minor breaking change for plugins, it can be left for 0.9 to avoid delaying 0.8 infinitely.