Removed one forgotten modifierFlags setter.

It’s not needed now the class is effectively immutable.
Refactored the shortcut dispatcher and bindings to user defaults.

This is a big change that was hard to split into smaller commits. There’s now
a new class to bind shortcuts to actions, a new class to bind user defaults’
keys to actions, and a new way to associate user defaults with the recorder
control (MASShortcutView). I have also updated the demo app to go with the
changes.

The new class to associate shortcuts with actions is called MASShortcutMonitor.
It wraps the Carbon hotkey magic and offers a simple interface to add a
shortcut along with a block that should be run when the shortcut is pressed.
It’s the lowest-level interface.

Since the usual requirement is to store the shortcuts into user defaults,
there’s also a higher-level interface offered by the MASShortcutBinder class.
That takes a defaults key and associates it with a block. When the shortcut
stored under the defaults key changes, the binder automatically switches to the
new shortcut. The class is a wrapper built atop of the previous one, the
MASShortcutMonitor – it simply adds, updates and removes shortcuts as the
user defaults change.

I have removed the special user defaults integration code from the recorder
control (MASShortcutView) and replaced it with a small Cocoa Bindings shim.
This means that in order to keep the recorder control in sync with the defaults
you just have to call the usual bind:toObject:withKeyPath:options: method,
like this:

[_shortcutView bind:MASShortcutBinding
    toObject:[NSUserDefaultsController sharedUserDefaultsController]
    withKeyPath[@"values.ExampleDefaultsKey"
    options:@{NSValueTransformerNameBindingOption:NSKeyedUnarchiveFromDataTransformerName}];

That’s more verbose than the previous solution, but it’s much cleaner and can
be swept under a convenience call if needed. I might also add a dictionaryValue
property later that would make it possible to bind the value to user defaults
directly, without a transformer, and would enable backward compatibility with
Shortcut Recorder.