Rewrite English.pot, add JXL image extension

[dcog989]

No description provided.

[sdottaka]

Thank you for the PR.
As for English.pot, this file is actually automatically generated from the Merge.rc file, so I had to modify Merge.rc. I will modify Merge.rc later.
Note that I may not incorporate all of the fixes in this PR.
For example, you have made a change to change many lowercase letters to uppercase. However, while this is fine for menu items, I would like the text displayed in the Options window to have only the first letter of a line in uppercase, like in other applications.

[dcog989]

Understood on capitalisation. I was just trying to be consistent everywhere.

If you like, reject this PR for the English.pot and I will resubmit with revised capitalisation applied to Merge.rc.

[sdottaka]

In commit 01df553, I merged the changes from English.pot into merge.rc and other files.

[dcog989]

I've skimmed all the changes. Looks good. Any issues, please let me know and I'll be happy to fix.