Re-add `Has*´ shortcuts

[Sharktheone]

We should re-add the old shortcuts from the Has* traits.

We want them so we better DX. For example instead of needing to write
<<<C as HasCssSystem>::CssSystem> as CssSystem>::Stylesheet we can write ::CssStylesheet`

and in some cases instead of
<<C::CssSystem> as CssSystem>::Stylesheet we can write C::CssStylesheet`

First we should evaluate what we actually need. And what might come additionally.

Afterwards we might consider adding a macro for getting rid of all of this syntactic hell. This could also serve as an alternative to the shortcuts.

[jaytaph]

Could we do something like:

pub type CssStylesheet<C> = <<C as HasCssSystem>::CssSystem as CssSystem>::Stylesheet;

Then we would have this:

fn stylesheets(&self) -> &[<C::CssSystem as CssSystem>::Stylesheet];
fn add_stylesheet(&mut self, sheet: <C::CssSystem as CssSystem>::Stylesheet);

and change it to:

use gosub_interface::config::types::CssStylesheet;

fn stylesheets(&self) -> &[CssStylesheet<C>];
fn add_stylesheet(&mut self, sheet: CssStylesheet<C>);

[jaytaph]

Links to #742

[Sharktheone]

There was a specific reason to not do that at the time of designing this initially. I think something was still unstable with this path, but I am not sure what.

This solution looks good!