Replace vector of UniformProperty with a single UniformProperty

[lachlansneff]

bevy_render::BindGroup::Uniform originally looked like this:

pub enum BindType {
    Uniform {
        dynamic: bool,
        properties: Vec<UniformProperty>,
    },
    ...
}

It turns out that properties doesn't need to be a vector, a struct is already a valid variant of UniformProperty.

So, this pr renames it to property and makes it a single item.

[lachlansneff]

I know you're extremely busy @cart, but it would be awesome to have this merged so I can start working on expanding the uniform types supported by bevy.

[cart]

Sure thing. This is an easy win!

I have one more thing to wrap up then I'm going to be working full time to work through the PR / issue backlog