splitview: hide handles when in single-pane mode

Prevents breaking the UI.

Gitlab: #1370
Change-Id: I6bad222c2a05dc66402bc8f86e17f9a6d4d99c99

src/app/commoncomponents/JamiSplitView.qml

@@ -68,6 +68,7 @@ SplitView {
     }
 
     handle: Rectangle {
+        visible: !isSinglePane
         implicitWidth: JamiTheme.splitViewHandlePreferredWidth
         implicitHeight: root.height
         color: JamiTheme.primaryBackgroundColor