Draft: Add a way to render animation from prerendered image sequence

This commit will add a checkbox under Video or Both option of the Render Animation Dialog that would allow rendering animation from already rendered image sequence.

• A checkbox "Use saved frames" to render video using already saved frames.
• This option will only be enabled if the last render was either video or both.
• It will not render the already saved frames.
• All files will be deleted if only video option is chosen.

Link to the request.

Test Plan

• Render a sequence in "Both" mode.(this should work for video render as well if there is a crash, and images are not deleted).
• Again render with "Use saved frames" option enabled, using some or all of the frames used in the previous render. Already saved frames would not be deleted. This should disable all the video and image options.
• If the previous render is Image sequence only this option is disabled.
• If rendering in the second step using Video only mode all files including previously existing files would be deleted.

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.