Add debug-widget-callstack and debug-interactive-widgets features

- they need to be separate, both on at once is too much

    --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

crates/notedeck_chrome/Cargo.toml

@@ -42,6 +42,8 @@ path = "src/preview.rs"
 [features]
 default = []
 profiling = ["notedeck_columns/puffin", "puffin", "puffin_egui"]
+debug-widget-callstack = ["egui/callstack"]
+debug-interactive-widgets = []
 
 [target.'cfg(target_os = "android")'.dependencies]
 android_logger = "0.11.1"

crates/notedeck_chrome/src/theme.rs

@@ -124,9 +124,26 @@ pub fn add_custom_style(is_mobile: bool, style: &mut Style) {
         ..Interaction::default()
     };
 
-    #[cfg(debug_assertions)]
+    // debug: show callstack for the current widget on hover if all
+    // modifier keys are pressed down.
+    #[cfg(feature = "debug-widget-callstack")]
     {
-        style.debug.show_interactive_widgets = true;
+        #[cfg(not(debug_assertions))]
+        compile_error!(
+            "The `debug-widget-callstack` feature requires a debug build, \
+             release builds are unsupported."
+        );
         style.debug.debug_on_hover_with_all_modifiers = true;
     }
+
+    // debug: show an overlay on all interactive widgets
+    #[cfg(feature = "debug-interactive-widgets")]
+    {
+        #[cfg(not(debug_assertions))]
+        compile_error!(
+            "The `debug-interactive-widgets` feature requires a debug build, \
+             release builds are unsupported."
+        );
+        style.debug.show_interactive_widgets = true;
+    }
 }