Add fixed_pos to Window

[tomassedovic]

This allows changing the position of a window by opening the window
with a different value.

[tomassedovic]

In my game, I want to have more control over how the windows work, e.g. control their size and position (e.g. cover the whole screen minus some padding), or disable collapsing (#15).

I'm not sure you want to go with the direction of extending the Window widget. I guess the alternative would be to write a custom widget, but since everything else would be the same, here's a couple PRs to see what you think.

[emilk]

Thanks for describing your use case!

I think it makes sense to add helpers like these to disable parts of the Window features. You won't be the only one to want to fix the position and size of windows.

If you don't even want the title bar you could consider just combining Frame and Area directly. Something like this:

Area::new("my_area").fixed_pos([32.0, 16.0]).show(ctx, |ui| {
    Frame::window(ui.style()).show(|ui| {
        ui.label("Hello world!");
    });
});

[emilk]

Please add a docstring (even though I have been lazy and not done so for the other functions)

[tomassedovic]

Thanks for showing the example of Area+Frame! In this particular usecase, I really want the UI, but I did have another one where I basically wanted a statically-positioned borderless window and wasn't sure how to do it nicely (ended up with a child Ui and a bunch of extra code).

[emilk]

Thanks for making the changes!