Hide "Scan a QR Code" with no camera connected

Also when you disconnect/turn off the camera while the QR scanner is
open, show a better error message instead of freezing the last frame.

src/app/qml/QrScannerPage.qml

@@ -27,9 +27,26 @@ Kirigami.Page {
         }
     }
 
+    MediaDevices {
+        id: devices
+    }
+
+    Rectangle {
+        anchors.fill: parent
+
+        color: Kirigami.Theme.backgroundColor
+        visible: devices.videoInputs.length === 0
+
+        Kirigami.PlaceholderMessage {
+            text: i18nc("@info", "No Camera Connected")
+            anchors.centerIn: parent
+        }
+    }
+
     VideoOutput {
         id: viewFinder
         anchors.centerIn: parent
+        visible: devices.videoInputs.length > 0
     }
 
     Prison.VideoScanner {
@@ -47,6 +64,8 @@ Kirigami.Page {
     }
 
     CaptureSession {
+        id: session
+
         camera: Camera {
             id: camera
         }

src/rooms/ExploreComponent.qml

@@ -6,6 +6,7 @@ pragma ComponentBehavior: Bound
 import QtQuick
 import QtQuick.Controls as QQC2
 import QtQuick.Layouts
+import QtMultimedia
 
 import org.kde.kirigami as Kirigami
 
@@ -25,6 +26,10 @@ RowLayout {
      */
     signal textChanged(string newText)
 
+    MediaDevices {
+        id: devices
+    }
+
     Kirigami.Heading {
         Layout.fillWidth: true
         visible: !root.collapsed
@@ -101,6 +106,7 @@ RowLayout {
             QQC2.MenuItem {
                 text: i18n("Scan a QR Code")
                 icon.name: "view-barcode-qr"
+                visible: devices.videoInputs.length > 0
                 onTriggered: (root.Kirigami.PageStack.pageStack as Kirigami.PageRow).pushDialogLayer(Qt.createComponent("org.kde.neochat", "QrScannerPage"), {
                     connection: root.connection
                 }, {