DefaultTheme shouldn't need to require Clone

[brandon-reinhart]

It seems possible to avoid this overly restrictive trait bound. In my case, I have a template type that I want to DefaultTheme. A template parameter is of a type that is unused in the type and cannot be Clone for reasons.

#[derive(Component, Reflect)]
pub(crate) struct ManifestEditor<M>
where
    M: Manifest + EditableManifest,
{
    manifest: String,
    handle: Handle<M::RawManifest>,
    loaded: bool,

    items: Entity,
    props: Entity,
}

impl<M> DefaultTheme for ManifestEditor<M>
where
    M: Manifest + EditableManifest,
{
    fn default_theme() -> Option<Theme<ManifestEditor<M>>> {
        ManifestEditor::<M>::theme().into()
    }
}

impl<M> ManifestEditor<M>
where
    M: Manifest + EditableManifest,
{
    pub const ITEMS: &'static str = "Items";
    pub const PROPS: &'static str = "Props";

    pub fn theme() -> Theme<ManifestEditor<M>> {
        let base_theme = PseudoTheme::deferred(None, ManifestEditor::<M>::primary_style);
        Theme::new(vec![base_theme])
    }

    fn primary_style(style_builder: &mut StyleBuilder, theme_data: &ThemeData) {}
}

It seems reasonable for this to be Sized. I tested this locally and the examples all work fine with this change.