dialogs: Update JPEG export dialog
This patch attempts to update the wording of the JPEG export dialogue section
- Uses of "JPG" and "jpeg" were replaced with "JPEG" for consistency
- The CMYK export option is now described
- Advanced Options were re-arranged to better match the order of the dialogue
- Minor edits to the subsampling description were made
@Wormnest Hi! Feel free to let me know what other changes you'd like.