Unfortunately, winit does not seem to create _Touch_ events for the touch pad
on my mac.  Only _TouchpadPressure_ events are sent.

Found some issues (like
[this](rust-windowing/winit#54)), but I am not sure
what they exactly mean:  Sometimes, touch events are mixed with
touch-to-pointer translation in the discussions.

The are a few open topics:
- egui_web currently translates touch events into pointer events.
  I guess this should change, such that egui itself performs this kind of
  conversion.
- `pub fn egui_web::pos_from_touch_event` is a public function, but I
  would like to change the return type to an `Option`.  Shouldn't this
  function be private, anyway?

InputState.touch was introduced with type `TouchState`, just as
InputState.pointer is of type `Pointer`.

The TouchState internally relies on a collection of `Gesture`s.  This commit
provides the first rudimentary implementation of a Gesture, but has no
functionality, yet.

So far, the method always returns `None`, but it should work as soon as the
`Zoom` gesture is implemented.

Although quite unlikely, it is still possible to connect more than one touch
device. (I have three touch pads connected to my MacBook in total, but
unfortunately `winit` sends touch events for none of them.)

We do not want to mix-up the touches from different devices.

The basic idea is that each gesture can focus on detection logic and does not
have to care (too much) about managing touch state in general.

a simple `TODO` should be enough

…esture-support

For now, it works for two fingers only.  The third finger interrupts the
gesture.

Bugs:
- Pinching in the demo window also moves the window -> Pointer events must be
  ignored when touch is active
- Pinching also works when doing it outside the demo window -> it would be nice
  to return the touch info in the `Response` of the painter allocation

…esture-support

…esture-support

This commit includes an improved Demo Window for egui_demo, and a complete
re-write of the gesture detection.  The PR should be ready for review, soon.

…esture-support

…esture-support

…esture-support

…dbe715b5b70433faacfe74b59c99e4a4#r623226656

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

…esture-support

…support