* Add `RequestedSize` and `RequestedPosition` enum to enable asking for
logical or physical size/position.
* Rename `Window::size()` to `Window::physical_size()`
* Make `Window::set_size(...)` take an `Into<RequestedSize>`
* Rename `Window::position()` to `Window::physical_position()`
* Make `Window::set_position(...)` take an `Into<RequestedPosition>`
* Change `WindowAdapter` and related classes to be able to handle
requests being made in the either physical or logical units.
Implement this for C++, Rust and node.
Fixes#909
Since we declare the struct in C++, we don't need to declare it in
rust for cbindgen anymore, as long as we expose the Size2D type from
euclid to the cbindgen_private namespace
