When using copyq's sync function, image items doesn't follow the subsequence

Before creating a new issue, see list of [known
issues](https://copyq.readthedocs.io/en/latest/known-issues.html).

**Describe the bug**
When using copyq's sync function, image items' position don't sync to another copyq instance.

**To Reproduce**
Steps to reproduce the behavior:
1. On machine A , open copyq's preference, go to "items" -> "sync", and set copyq sync to a local folder (shared with machine B).
2. On machine B , open copyq's preference, go to "items" -> "sync", and set copyq sync to the same folder.
3. Create some items with at least one image item in copyq's item list.
4. On machine A, drag and drop a image item to change its position in copyq's item list.
5. On machine B, the image item doesn't appear in the same position as the image item on machine A.

**Expected behavior**
A clear and concise description of what you expected to happen.
On machine B , the image item appear in the same position as the image item on machine A.