plug-ins: various string fixes

Some small string fixes nice to get out of the way before the 3.0 string freeze.

• GIMP Online used [ as an accelerator. It seems the brackets were added for sorting, so instead use the new section syntax to put the TOC first. Fixes #9903 (closed). Also fix accelerator collisions at the same time.
• Fix a 15 year old naming collision in GIMP Online that prevented the "Use Selections" entry from being shown.
• Remove an _ in the titlebar for the file picker of Histogram export.
• Fix wording in fractal-explorer tooltip, the options are above as already said in the corresponding label.

v2: Change gimp-online fixes to also take care of #9903 (closed)