Follow the System Theme in egui (#4860)

* Some initial progress towards #4490

This PR just moves `Theme` and the "follow system theme" settings to
egui and adds `RawInput.system_theme`.
A follow-up PR can then introduce the two separate `dark_mode_style` and
`light_mode_style` fields on `Options`.


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### Breaking changes

The options `follow_system_theme` and `default_theme` has been moved
from `eframe` into `egui::Options`, settable with `ctx.options_mut`

---------

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

crates/egui_glow/examples/pure_glow.rs

@@ -192,7 +192,8 @@ impl winit::application::ApplicationHandler<UserEvent> for GlowApp {
         let gl = std::sync::Arc::new(gl);
         gl_window.window().set_visible(true);
 
-        let egui_glow = egui_glow::EguiGlow::new(event_loop, gl.clone(), None, None, true);
+        let egui_glow =
+            egui_glow::EguiGlow::new(event_loop, &gl_window.window, gl.clone(), None, None, true);
 
         let event_loop_proxy = egui::mutex::Mutex::new(self.proxy.clone());
         egui_glow

crates/egui_glow/src/winit.rs

@@ -24,6 +24,7 @@ impl EguiGlow {
     /// For automatic shader version detection set `shader_version` to `None`.
     pub fn new(
         event_loop: &winit::event_loop::ActiveEventLoop,
+        window: &winit::window::Window,
         gl: std::sync::Arc<glow::Context>,
         shader_version: Option<ShaderVersion>,
         native_pixels_per_point: Option<f32>,
@@ -42,6 +43,7 @@ impl EguiGlow {
             ViewportId::ROOT,
             event_loop,
             native_pixels_per_point,
+            window.theme(),
             Some(painter.max_texture_side()),
         );