This is a good early-out for widgets in `ScrollAreas`, but
also prepares for speeding up the first pass of a possible two-pass
version of egui: https://github.com/emilk/egui/issues/843
This enables functions that return `impl Widget`, so that you can
create a widget by just returning a lambda from a function.
For instance: `ui.add(toggle(bool))` (instead of `toggle(ui, bool)`)
Add support for primary, secondary and middle mouse buttons. Also improve ability to click things in low FPS situations.
This introduces a lot of breaking changes:
Backends/integrations now pass mouse events via the even stream.
Response has an interface of mostly methods instead of public members.
input.mouse is now input.pointer and has new interface.
* Rename 'mouse' to 'pointer' everywhere (pointer = mouse or touch)
* Make Response::clicked and Response::double_clicked into methods
* Remove Response::active and add dragged() and interact_pointer_pos()
* Support multiple mouse buttons
* Make PointerState interface all methods
* Make most members of Response private
