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

src/app_style.rs

@@ -69,6 +69,19 @@ pub fn create_custom_style(ctx: &Context, font_size: fn(&NotedeckTextStyle) -> f
         ..Interaction::default()
     };
 
+    // debug: show callstack for the current widget on hover if all
+    // modifier keys are pressed down.
+    #[cfg(feature = "debug-widget-callstack")]
+    {
+        style.debug.debug_on_hover_with_all_modifiers = true;
+    }
+
+    // debug: show an overlay on all interactive widgets
+    #[cfg(feature = "debug-interactive-widgets")]
+    {
+        style.debug.show_interactive_widgets = true;
+    }
+
     style
 }