add theme titlebar height and app-owned strip dragging

crates/theme/src/system.rs

@@ -78,6 +78,8 @@ pub struct SystemTheme {
     pub control_radius: Option<Pixels>,
     /// The platform's stated text-field height, if it has one.
     pub input_height: Option<Pixels>,
+    /// The platform's native titlebar height, if it states one.
+    pub titlebar_height: Option<Pixels>,
     /// The platform's native root font size, if it has one.
     pub root_font_size: Option<Pixels>,
     pub accessibility: AccessibilityPreferences,
@@ -92,6 +94,7 @@ impl SystemTheme {
             palette: PartialPalette::default(),
             control_radius: None,
             input_height: None,
+            titlebar_height: None,
             root_font_size: None,
             accessibility: AccessibilityPreferences::default(),
             source: SourceKind::Fallback,