Improve inputs

[bdlukaa]

Currently, several input widgets uses a HoverButton as backend. Currently, it only supports one button state at the time, but it should support multiple. For example, when pressing and hovering, it should provide both to the builder callback.

[bdlukaa]

I think we should remove the HoverButton widget, and implement the mouse, gesture detector and focus directly on each input. This would add more boilerplate code, but would give us more control over these things in each widget. This is the approach used on material and cupertino libraries.