Show warning for "clear" button

[nicebread]

The button is dangerous: A single click can destroy all of your data if you did not export it.

Please add a warning: "Warning: This will delete all of your local data. Do you want to export the data to a file?"

--> two buttons in modal:

• "Cancel"
• "Export data to local file"
• "Clear all data" (<- in red)

[alpkaanaksu]

Implemented here 56bd40d79d00faa4e654cec97538d5ce0943bbcc