[Media Controller] Use double for length instead of int

Length is in microseconds, so watching a long video (in excess of 33.3 minutes) will break the slider.

CHANGELOG: Media Controller can now properly handle and seek long tracks (> 30 minutes)

BUG: 377623
FIXED-IN: 5.8.7

applets/mediacontroller/contents/ui/ExpandedRepresentation.qml

@@ -35,7 +35,7 @@ Item {
     readonly property int controlSize: Math.min(height, width) / 4
 
     property int position: mpris2Source.currentData.Position || 0
-    readonly property int length: currentMetadata ? currentMetadata["mpris:length"] || 0 : 0
+    readonly property double length: currentMetadata ? currentMetadata["mpris:length"] || 0 : 0
 
     property bool disablePositionUpdate: false
     property bool keyPressed: false