Add "Lock/Unlock editing" toggle button for individual note

[dts-1990]

• Lock to disable editing note's content and hide cursor
• Store locked when toggled & restore locked state when restart the app

[dts-1990]

This would address https://github.com/linuxmint/sticky/issues/120

[clefebvre]

I don't see the need for this. We've the ability to undo changes just like in any other app which can act as a text editor, and that lock feature isn't something we see in other editors either. The same argument could be made with xed.