Improve Frame API to allow picking color until after adding content

[emilk]

Previously the Frame API only supported picking colors once, and then adding widgets.

But what if you want to add widgets first, and THEN pick the colors for the frame? Now you can!

let frame = Frame::default().inner_margin(4.0).begin(ui);
{
    frame.content_ui.label("Inside the frame");
    frame.content_ui.label("This too");
}
let response = frame.allocate_space(ui);
if response.hovered() {
    frame.frame.stroke = Stroke::new(2.0, Color32::WHITE);
}
frame.paint(ui);