Comment/uncomment on non-US keyboards

I use a Spanish keyboard.

Next to the menu item Edit->Comment/Uncomment, the hint for the corresponding keyboard shortcut is
```
Ctrl+Slash
```

On the Spanish keyboard, there is no Slash key. The slash character is done with Shift+7.

I have tried both Ctrl+7 and Ctrl+Shift+7 and neither works.

So either the shortcut is not compatible with all non-US keyboards, or the shortcut is not correctly indicated in the menu.

Reproduce Edit→Comment/Uncomment in the Arduino IDE with a Spanish keyboard, comparing the displayed Ctrl+Slash hint with Ctrl+7 and Ctrl+Shift+7. Trace the shortcut handling and menu-labeling entry points; done means commenting works with the supported Spanish-keyboard shortcut and the menu hint matches it.

Written by the indexing model from the issue text.