Create a background task for XMP startup update

[quazardous]

Is your feature request related to a problem? Please describe.

For large photos libraries, turning on the update XMP at startup feature can delay the DT splash screen and main window.

It can be very confusing and can lead to try to open another DT (wich will be bocked by DB lock).

Describe the solution you'd like

The XMP startup update could occur after the main window is open and ready and be handled by a background task with a progression (or something) in the status bar.

Ideally a flag on photos not yet checked could be usefull.

Alternatives
If background task is too complicated, a progression modal could be shown before the main window appears.

Additional context

It can be related to #14686

[github-actions]

This issue has been marked as stale due to inactivity for the last 60 days. It will be automatically closed in 300 days if no update occurs. Please check if the master branch has fixed it and report again or close the issue.

[ralfbrown]

#17202 will provide an updating progress report on the sidecar check.

[github-actions]

This issue has been marked as stale due to inactivity for the last 60 days. It will be automatically closed in 300 days if no update occurs. Please check if the master branch has fixed it and report again or close the issue.