Improve demos for edge-to-edge mode

Set root window background correctly and add safe margins to
QML Window when needed so that demo apps look good also on
edge-to-edge mode.

Ideally, we could also set Qt.ExpandedClientAreaHint window
flag, but since Android doesn't handle status bar coloring
well yet, it's better to leave it until that's supported.

Change-Id: I2bf55aaa990909dea340c195056afcd511960395
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
(cherry picked from commit 5a5d0e5020)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
(cherry picked from commit 0020b1f825)

examples/demos/calqlatr/Main.qml

@@ -20,6 +20,11 @@ Window {
         id: root
         anchors.fill: parent
 
+        anchors.topMargin: parent.SafeArea.margins.top
+        anchors.leftMargin: parent.SafeArea.margins.left
+        anchors.rightMargin: parent.SafeArea.margins.right
+        anchors.bottomMargin: parent.SafeArea.margins.bottom
+
         readonly property int margin: 18
         readonly property color backgroundColor: "#222222"
         readonly property int minLandscapeModeWidth: numberPad.landscapeModeWidth

examples/demos/coffee/main.qml

@@ -8,6 +8,11 @@ ApplicationWindow {
     width: 1000
     height: 600
     title: qsTr("Coffee")
+
+    background: Rectangle {
+        color: Colors.currentTheme.background
+    }
+
 //! [Set application window size]
     ApplicationFlow {
         width: parent.width