Link tags to non-existing items are silently removed after saving taxts

[jonkerz]

See #177.

[jonkerz]

I'll not have time to work on this, but note that the usual check for detecting invalid ref codes is still in place and active, so it's not possible to save or add an item containing {xX9f}: 188 (Europe species key);. However, the same text can be saved when it's followed by other tags like here: {xX9f}: 188 (Europe species key); {Forel, 1874 xjpf}: 84 (Switzerland species key);.