Make empty annotations disappear automatically

[MorrowShore]

This would cause empty annotations to be removed as soon as they're deselected, or as soon as another tool is summoned.

This would greatly reduce clutter, and allow an option from the edit menu to be removed.

[askmeaboutlo0m]

We can't remove the option from the edit menu because empty annotations will still be left strewn about from old clients or in case they get disconnected after creating an empty annotation. The latter case may be pathological enough to ignore, but the former is gonna continue happening, at least for now.

[amoruwu]

I really like this idea! Some boards have turned off annotations just because people forget empty ones. Very Neat Idea!