Allow changing ProgressBar fill color

[Uriopass]

Here's with the demo with

        let progress_bar = egui::ProgressBar::new(progress)
            .fill(egui::Color32::from_rgb(0, 100, 0))
            .show_percentage()
            .animate(*animate_progress_bar);

[Uriopass]

The CI failure doesn't look related to this PR :/

failures:

---- src/ui.rs - ui::Ui::menu_image_button (line 2166) stdout ----
error: couldn't read src/rss.svg: No such file or directory (os error 2)
Error:  --> src/ui.rs:2171:5
  |
8 |     include_bytes!("rss.svg"),
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: this error originates in the macro `include_bytes` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0432]: unresolved import `egui_extras`
Error:  --> src/ui.rs:2167:5
  |
4 | use egui_extras;
  |     ^^^^^^^^^^^ no external crate `egui_extras`

error[E0433]: failed to resolve: use of undeclared crate or module `egui_extrasRetainedImage`
Error:  --> src/ui.rs:2169:11
  |
6 | let img = egui_extrasRetainedImage::from_svg_bytes_with_size(
  |           ^^^^^^^^^^^^^^^^^^^^^^^^ use of undeclared crate or module `egui_extrasRetainedImage`

error[E0425]: cannot find value `ui` in this scope
Error:   --> src/ui.rs:[217](https://github.com/emilk/egui/actions/runs/3985584382/jobs/6833804530#step:11:218)5:1
   |
12 | ui.menu_image_button(img.texture_id(ctx), img.size_vec2(), |ui| {
   | ^^ not found in this scope

error[E0425]: cannot find value `ctx` in this scope
Error:   --> src/ui.rs:2175:37
   |
12 | ui.menu_image_button(img.texture_id(ctx), img.size_vec2(), |ui| {
   |                                     ^^^ not found in this scope