Issues with bullets_delete_last_bullet_if_empty = 2

[mario-holubar]

bullets_delete_last_bullet_if_empty = 2 has several issues:

1. Checkbox doesn't get removed properly

See #165.

2. It causes two undo actions

Undo has to be pressed twice to revert back to the state before pressing <CR>.

3. Checkbox reverts to bullet if a mix of both is used

Pressing <CR> here causes the checkbox to disappear:

- a
    - [ ] |

I could see this becoming an intended behavior if it also worked the other way around: matching the type of the bullet at the new level when promoting. Would be nice to have an option for this.

EDIT: Both 1 and 3 are actually issues with promotion behavior in general, not the bullets_delete_last_bullet_if_empty setting.