Prepare Image & Video loading for E2EE

Changes the urls to make sure they are decrypted, while making sure that
it is backwards compatible to libQuotient 0.6

imports/NeoChat/Component/FullScreenImage.qml

@@ -9,8 +9,8 @@ import org.kde.kirigami 2.15 as Kirigami
 ApplicationWindow {
     id: root
 
+    property alias source: image.source
     property string filename
-    property url localPath
     property string blurhash: ""
     property int imageWidth: -1
     property int imageHeight: -1
@@ -45,8 +45,6 @@ ApplicationWindow {
 
         fillMode: Image.PreserveAspectFit
 
-        source: localPath
-
         Image {
             anchors.centerIn: parent
             width: image.width