Adding the ability to change the mouse cursor for MacOS and Windows

[discordance]

The feature was already there for X11, but wasn't available in MacOS and Windows.

The code is based on winit code mostly. I've tested locally with a modified version of iced_baseview that I am working on for a project. Not sure if adding a set_mouse_cursor to the window is the way to go but it works for me.

This is related to the #123 issue.