WindowServer: New title bar vars for themes

The theming system can now control title bar height, title button
size, title stripe color and the title text shadow color.
The implemented theme metrics system could be later extended to LibGUI
to allow themes to change widget padding, border width, etc.

Libraries/LibGfx/SystemTheme.h

@@ -47,6 +47,8 @@ enum class ColorRole {
     HighlightWindowBorder1,
     HighlightWindowBorder2,
     HighlightWindowTitle,
+    WindowTitleShadow,
+    WindowTitleStripes,
     MenuStripe,
     MenuBase,
     MenuBaseText,
@@ -95,8 +97,17 @@ enum class ColorRole {
     DisabledText = ThreedShadow1,
 };
 
+enum class MetricRole {
+    NoRole,
+    TitleHeight,
+    TitleButtonWidth,
+    TitleButtonHeight,
+    __Count,
+};
+
 struct SystemTheme {
     Color color[(int)ColorRole::__Count];
+    int metric[(int)MetricRole::__Count];
 };
 
 const SystemTheme& current_system_theme();