exepta/bevy_extended_ui

AI Summary: Implement support for the CSS `:placeholder` pseudo-class in the bevy_extended_ui crate to allow users to customize the color and appearance of input field placeholders using CSS.

AI Summary: Debug and fix a bug in the slider widget of the bevy_extended_ui crate. The issue is with the incorrect calculation of the slider track and thumb positions, likely due to problems with `global_transform` and `Pointer<Click>` event handling. A better logic for handling these elements is needed.

AI Summary: Implement a new `ProgressBar` widget for the `bevy_extended_ui` crate. The widget should visually represent progress, similar to a slider but updated via events or triggers rather than user interaction. It should integrate seamlessly with the existing CSS styling system.