This commit adds an `EventLoopBuilder` struct to simplify event loop
customization and providing options to it upon creation. It also
deprecates the use of `EventLoop::with_user_event` in favor of the same
method on new builder, and replaces old platforms specific extension
traits with the new ones on the `EventLoopBuilder`.
* Change web backend `event_handler` to without 'static lifetime
* Refactor web runner and fix ControlFlow::Exit not being sticky
* Impl. scaling change event for web-sys backend
* Improve `dpi` docs regarding the web backend
* Add changes to changelog
* Update features.md
PhysicalSize is recorded as canvas.size, whereas LogicalSize is stored
as canvas.style.size.
The previous cursor behavior on stdweb clobbered all style - thus losing
the LogicalSize.
* fix: use a 'static lifetime for the web backend's `Event` types
* implement hidpi for stdweb (web-sys wip?)
* fix: make all canvas resizes go through backend::set_canvas_size
* update Window docs for web, make `inner/outer_position` return the position in the viewport
