Fix loading m.image events in encrypted rooms

Fix bug introduced in 11fb32:

When loading an encrypted thumbnail, QML lacks the decryption dict for
half a second at first.

When calling python like this, python calls the wrong matrix API
for fetching the encrypted thumbnail, and the added retry code
would be forever stuck.

The retry code has been moved to QML, and now works for all HImage.

src/gui/Base/HAvatar.qml

@@ -83,7 +83,6 @@ Rectangle {
             )
 
             visible: ! avatarImage.broken &&
-                     avatarImage.status !== Image.Error &&
                      avatarImage.width < dimension * 0.75 &&
                      (toolTipSourceOverride || toolTipMxc) &&
                      hoverHandler.hovered