Support custom window icon

[dhardy]

API: https://docs.rs/winit/0.20.0/winit/window/struct.Icon.html

[dhardy]

Probably the best approach is to add a method providing the icon data to the kas::Window trait, then use that within kas_wgpu when opening the window.

[dhardy]

We now have image_from_path(path) -> Result<ImageId, ..>. This is used for drawing images via the shell, thus uploads the decoded image to GPU memory and does not make it available through the API.

We should allow re-use of the image loading code in the form of a resource manager, updating the image code to use that:

// with singleton instances for T == Image, ...
ResourceManager<T> {
    // deduplicate path(?) and load:
    fn load(&mut self, path: PathBuf) -> Result<ResId, Error>;
    fn free(...);
    fn get(&self, id: ResId) -> Option<&T>;
}

[dhardy]

In this instance it isn't worth worrying about deduplication, though I did add a TODO(opt) comment to #228.