Support for multiple Windows

[hansl]

• Version of embedded-graphics-simulator in use (if applicable): 0.5.0
• Version of embedded-graphics in use (if applicable): latest

Description of the problem/feature request/other

I have a device with two displays and I wanted to use the simulator as frontend for desktops, and framebuffer for the device itself (removing the with-sdl flag when building embedded).

Because of the two displays, I wanted use two Window objects, but I of course I got this error:

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: "Cannot initialize `Sdl` more than once at a time."'

This doesn't seem to be very hard to fix, at least for my simple use case. There's already a PR (#37). I just wanted to file an issue to track progress of this.

Test case (if applicable)

        let window_osd = Window::new(
            "OSD",
            &OutputSettingsBuilder::new()
                .theme(BinaryColorTheme::OledBlue)
                .build(),
        );

        let window_title = Window::new(
            "Title",
            &OutputSettingsBuilder::new()
                .theme(BinaryColorTheme::OledBlue)
                .build(),
        );