cuicui_layout integration for bevy_ui

Provides the UiDsl trait, extending the cuicui_layout's LayoutDsl with the following features:

• Style border color and width (but only uniform)
• UI node background color
• UI node background image
• Direct spawning of text and images as argument to spawn_ui.

Note that this is in addition to the methods LayoutDsl already supports.

Additionally, cuicui_layout_bevy_ui provides a ContentSized implementation that supports Text and UiImage terminal nodes which size is determined by their content.

Check the following crates for details:

• cuicui_layout: the layouting algorithm
• cuicui_dsl: the dsl! macro and what UiDsl entails.