Platform specific window options.

[casperstorm]

This PR enables us to add some platform specific window options.
For macOS it means we can set three new variables:

    /// Hides the window title.
    pub title_hidden: bool,
    /// Makes the titlebar transparent and allows the content to appear behind it.
    pub titlebar_transparent: bool,
    /// Makes the window content appear behind the titlebar.
    pub fullsize_content_view: bool,

An example below is shown where all three of the above mentioned variables are enabled:

Settings {
        window: iced::window::Settings {
            platform_specific: iced::window::PlatformSpecific {
                title_hidden: true,
                titlebar_transparent: true,
                fullsize_content_view: true,
            },
            ..Default::default()
        },
    ..Default::default()
}

[hecrj]

Thank you!

I think we can expose the iced_winit struct directly, since the platform specific settings are already internal details. I did this in the last commit.