Platform: handle (multi-)touch events in EmscriptenApplication.

The impossible-to-reliably-disable behavior with compatibility mouse
events is quite a headache. I wish Emscripten implemented pointer events
already so I could ditch this mess -- especially the array of 32 touches
where all of them but one will be unchanged is stupid.

For the internals unfortunately, EmscriptenMouseEvent and
EmscriptenTouchEvent have no common base, so I had to give up on the
current way of querying the event struct directly from event getters, as
that'd be too nasty with the branching and casts. Instead the relevant
fields are put directly into the events themselves.

HTML5 also doesn't provide any relative pointer position. For the mouse
it was rather straightforward, but for the up-to-32 touches I have to
maintain an array of per-finger positions and match them by ID.
Hopefully the linear lookup is fine. I'll probably use the same approach
for the AndroidApplication.

doc/changelog.dox

@@ -339,10 +339,14 @@ See also:
     @relativeref{Platform::GlfwApplication,tickEvent()} to match the interface
     of @ref Platform::Sdl2Application (see [mosra/magnum#577](https://github.com/mosra/magnum/issues/577)
     and [mosra/magnum#580](https://github.com/mosra/magnum/pull/580))
--   Multi-touch support in @ref Platform::Sdl2Application through new
+-   Multi-touch support in @ref Platform::Sdl2Application and
+    @ref Platform::EmscriptenApplication through new
     @relativeref{Platform::Sdl2Application,PointerEvent} and
     @relativeref{Platform::Sdl2Application,PointerMoveEvent} that unify mouse
-    and touch input events
+    and touch input events. This also means @ref Platform::EmscriptenApplication
+    finally supports touch drag ([mosra/magnum#532](https://github.com/mosra/magnum/issues/532))
+    --- no touch-to-mouse event passthrough needs to be implemented, it works
+    out of the box with the new pointer events.
 
 @subsubsection changelog-latest-new-scenegraph SceneGraph library