[2.13] Check hotkey conflicts case-insensitive
This is needed to catch conflicts between uppercase and lowercase
hotkeys of the same letter, as seen with
the german utils translations.
(cherry picked from commit 07bd1139)
Also fix hotkey conflict in utils id.po and sv.po (cherry picked from commit 7cf54f2c)
Note that 7cf54f2c also included fixes for de.po which are not needed in the 2.13 branch.
This is the 2.13 variant of !675 (merged).