Access to higher-fidelity events for TextField

At the moment, `TextField` only supports the `OnChanged` event callback, which triggers every time the user makes an edit to the field (often just a single character change). It is hard to build useful features on top of this, because you do not know if the user pressed a key, pasted, released a key, erased a character, etc. Furthermore, you cannot detect actions such as the user pressing the enter key in the field, because this does not change the contents of the text field. Having a finer-grained set of events for `TextField` (`keyUp` and `keyDown` come to mind) would be extremely useful, and would enable much richer interactions with input fields.