Button types should have Event parameter as value

[jlstevens]

Currently panel Buttons don't have a .value parameter like most other widgets. Instead, they have a clicks parameter that increments on each click.

With the addition of Event parameters in param, there is now a parameter that can represent transient events such as button clicks. The behavior is that the boolean value is True while callbacks are being processed post click and False otherwise. This lets you know which button was clicked if you have a callback that can be triggered by multiple different buttons.

I think this addition would help make panel buttons more consistent with other panel widgets and it would also help make buttons play nicely with other libraries (HoloViews in particular).

[jlstevens]

Thanks!