Make all input components consistent (in terms of events, parameters, etc)

[Aran-Fey]

In terms of interface:

• Focus events: on_gain_focus, on_lose_focus
• Change event: on_change
• Confirm event: on_confirm

There are also some design decisions we should think about:

• Should each component get its own events? Do we really need a TextInputFocusEvent and a MultiLineTextInputFocusEvent and a NumberInputFocusEvent and ...?
• Should there be a base class (in JS and/or in python) that implements this interface?

In terms of optimization, we should also avoid sending unnecessary messages between the backend and the frontend. Most TextInputs don't have a on_gain_focus or on_lose_focus event handler, and yet the backend notifies the frontend every time one of these events occurs.