feat: add reusable AppWindow with theme defaults

This will simplify setup for any application without having to specify
palette.

AppWindow.qml

@@ -0,0 +1,23 @@
+import QtQuick
+import QtQuick.Controls.Basic
+
+ApplicationWindow {
+    id: root
+    visible: true
+
+    palette {
+        accent: Theme.accent
+        dark: Theme.foreground
+        window: Theme.background
+        windowText: Theme.foreground
+        buttonText: Theme.foreground
+        base: Theme.muted
+        brightText: Theme.muted
+        text: Theme.foreground
+        button: Theme.accent
+    }
+
+    title: qsTr("Flowy")
+    width: 640
+    height: 480
+}

CMakeLists.txt

@@ -19,6 +19,7 @@ set(plain_qml_files
   AppSubHeadline.qml
   AppLabel.qml
   AppLogo.qml
+  AppWindow.qml
 )
 set(qml_singletons
   Fonts.qml

gallery/Main.qml

@@ -5,7 +5,7 @@ import DesignSystem
 import QtQuick.Layouts
 import QtQuick.Controls.Basic
 
-ApplicationWindow {
+AppWindow {
     id: window
     width: 640
     height: 480