Adding Demo to facilitate debug

[miku1958]

By forwarding the keyDown event, you can directly debug IMKInputController in an NSTextView.

To do this, we need to:

1. Adding IMKInputController and related files to an individual framework.
2. Separate SquirrelApplicationDelegate to make rime work properly.
3. Create a custom IMKTextInput client
4. Disable super.init(server: server, delegate: delegate, client: client) as Custom IMKTextInput Client not recognized by AppKit
5. Forwarding text changes to textView

Demo: