Prevents this: user paste image (image provider retrieves the clipboard
image to show it as a thumbnail),
and immediatly presses enter (retrieves the image to convert it and send
to Python); the two calls would run at the same time instead of one
waiting for the other to set the cache.
Will be used to show a preview before uploading image from clipboard.
Transforming the clipboard image to PNG (or any other real format) is
slow and freezes the GUI, with the provider we can display it directly.
