debug: add crate features which enable egui DebugOptions

--features debug-widget-callstack
  Show callstack for the current widget on hover if all modifier keys
  are pressed down

--features debug-interactive-widgets
  Show an overlay on all interactive widgets

Notes:
- debug-widget-callstack asserts `egui:callstack` feature when enabled
- Only works in debug builds, compile error w/ release builds

Cargo.lock

@@ -1112,6 +1112,7 @@ source = "git+https://github.com/emilk/egui?rev=fcb7764e48ce00f8f8e58da10f937410
 dependencies = [
  "accesskit",
  "ahash",
+ "backtrace",
  "emath",
  "epaint",
  "log",