feat: jsonrpc: add `late_file_message_mediasize`

[Simon-Laux]

[link2xt]

I don't understand the purpose of this PR. If the core is buggy and provides wrong dimensions, UI has to check and correct it, we can as well just drop the API to read these values and move the logic entirely in the UI instead of the UI telling the core what it should tell the UI.

[r10s]

the idea of the api was to speed up things on subsequent loads on android, eg UI can show placeholder in correct aspect ratio, leading to less flickering and jumping, esp if there are lots of images/videos

but that was 7 years ago, if desktop could live without the api up to now, there might be no need :)

for android, we can check that at some point, if it is still needed, i would not remove the api just now

[Simon-Laux]

closing for now, desktop found a different solution, we can still re-open or redo should there be a need in the future.