Master: Ensure that launched commands only appear once in the history.

Currently the appfinder always adds a started command to the history, even if the command was already in the history. That leads to a growing history whenever an already launched command is launched again:

The tiny patch avoids duplicate entries and checks the hash table model->items_hash for presence of a command. The command is not added to the hash (model->items_hash) or list (model->items) if the command is already present.

Please review and merge if acceptable.