Adding utility to center windows in their parent container.

src/el/window.cc

@@ -180,6 +180,14 @@ void Window::set_settings(WindowSettings settings) {
   Invalidate();
 }
 
+void Window::CenterInParent() {
+  if (!parent()) {
+    return;
+  }
+  Rect bounds(0, 0, parent()->rect().w, parent()->rect().h);
+  set_rect(rect().CenterIn(bounds).MoveIn(bounds).Clip(bounds));
+}
+
 int Window::title_bar_height() {
   if (any(window_settings_ & WindowSettings::kTitleBar)) {
     return title_mover_.GetPreferredSize().pref_h;

src/el/window.h

@@ -77,6 +77,8 @@ class Window : public Element {
   // Sets settings for how this window should look and behave.
   void set_settings(WindowSettings settings);
 
+  void CenterInParent();
+
   // ResizeFit specifies how ResizeToFitContent should resize the window.
   enum class ResizeFit {
     kPreferred,        // Fit the preferred size of all content.