Simplified bindings support for SRRecorderControl

This is a smaller subset of the code bundled in [#23](https://github.com/Kentzo/ShortcutRecorder/pull/23), just the simplified bindings code. It should be backwards compatible: the overriden `bind:toObject:withKeyPath:options:` method is gone, but I have added a new `value` property (backed by `objectValue`) to keep the `NSValueBinding` working.

I have not tested the code, because I would have to clone the demo project, make the changes there and then port them to my fork of the Shortcut Recorder. (The submodule points to the official fork, which prevents me from simply pushing the changes upstream, doesn’t it?) That’s too much of a hassle for me, please consider making the demo a part of the main project.