Reverting "view image/video" behaviour in context menus to open in current tab

[wyatt8740]

Currently the code only relabels the context menu entries for english locales, but i took some extra time to write the function in a way that it should be easy to add support for more languages in the future as needed. Wasn't sure how best to deal with locale troubles.

The functionality part works in all languages, just the labels in the menus may still say "open image in new tab" in non-english locales even though it opens in current tab. It is tested in Firefox 133 (debian sid), en-US.

The source-level equivalent diff (which also does some other things) for firefox 129 can be found at http://megatokyo.moe/dl/moz/129/diff-129-with-extra-rust-fix.patch