Ask for removal of a non existing game

[walkero-gr]

#147 introduced a check if the slave exists. If not a message is shown to the user. Now we need to ask the user if he wants the entry to be removed from the list. This way, a non existing entry can be removed from the list, which should be saved automatically.

This should be done after #149 and use the same methods for the entry removal.
Part of this work is to update the guide file, if needed, to describe the new feature.