Fix for Remove object on linux

Should work better with frames and iframes.
Works on linux again.

Added keyboard shortcut Alt+Shift+x to trigger the Nuke on [x] mode.
Added new languages

- Icon is a bit more gray-scale to be less distracting.
- Clicking on "Toggle Nuke on [x] mode" sub-menu a second time goes back to normal as if you clicked on the [Esc] key
- "Nuke no [x] mode" can be used with the mouse by Ctrl-click on any object to remove, as if you clicked on [x]
- Added settings page to allow selecting which items to display in the sub-menu. If you select only *one* item to display then there it is displayed in the menu without a sub-menu. (Workaround for Firefox Web Extension limitation)

New in this version " Activate Nuke on [x] key "

Which allows you to remove as many elements as you want by repeatedly pressing the [X] key on the keyboard.

The [U] key will undo last action.
The [Esc] Key will exit this mode and go back to normal.