Prevent a crash caused by empty emoji

The crash happens because the serialization of recently used emoji uses
';' to separate the emojis.
One of the ASCII emojis however has a ';' in the beginning.
When this one is used by the user,
it causes an empty string to be returned when deserializing.

This commit prevents the crash by changing the separator to a tab.
It uses a different settings string to store the emoji,
so users will lose the list of recently used emoji when they update to
this version.

PS. That wasn't my idea ;)