Follow the system theme in egui

[bash]

• Some initial progress towards Custom light & dark themes on system theme change #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.

• I have followed the instructions in the PR template

Breaking changes

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

[emilk]

• Upgrade winit to 0.30.2 #4849 creates a bunch of merge conflicts that needs to be resolved

[emilk]

Code looks very good; I'll test once the winit 0.30 changes has been merged in

[bircni]

Looks like some doc comments are still missing:

warning: missing documentation for a method
  --> crates/egui/src/memory/theme.rs:13:5
   |
13 |     pub fn default_visuals(self) -> crate::Visuals {
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
note: the lint level is defined here
  --> crates/egui/src/memory.rs:1:9
   |
1  | #![warn(missing_docs)] // Let's keep this file well-documented.` to memory.rs
   |         ^^^^^^^^^^^^

warning: missing documentation for an associated function
  --> crates/egui/src/memory/theme.rs:20:5
   |
20 |     pub fn from_dark_mode(dark_mode: bool) -> Self {
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

warning: `egui` (lib) generated 2 warnings

[bash]

Looks like some doc comments are still missing:

Ah yes, I committed the review suggestions without thinking about this 😅 I'll fix it when I rebase :)

[bash]

winit 0.30 changes are merged in 🎉

[emilk]

Very nice, works great!