Restructure shortcut keys across the program

- Centralize customizations in a common class
- Ensure menu items list accelerators registered in the input map
- Add Ctrl+7 shortcut for toggle comments for DE/ES/PT, etc. keyboards 
- Make it easier to override default shortcuts 
(see https://forum.image.sc/t/shiny-new-script-editor/64160/11?)

Author: tferr