callview: change preview width and set minimum value

Gitlab: #104
Change-Id: I13414206cecc548b3ff481f4f0f79574cf0b4a4b

src/constant/JamiTheme.qml

@@ -110,6 +110,7 @@ Item {
     property int preferredMarginSize: 16
     property int preferredDialogWidth: 400
     property int preferredDialogHeight: 300
+    property int minimumPreviewWidth: 120
 
     // Misc.
     property color white: "white"

src/mainview/components/VideoCallPage.qml

@@ -241,7 +241,7 @@ Rectangle {
                         }
                     }
 
-                    width: videoCallPageMainRect.width / 4
+                    width: Math.max(videoCallPageMainRect.width / 5, JamiTheme.minimumPreviewWidth)
                     height: {
                         previewImageScalingFactorUpdated
                         return previewRenderer.width * previewRenderer.getPreviewImageScalingFactor()