Automatic reload when the database get externally modified

Feature: if the .kdbx database get modified by an other instance of Keepassx, Keepassx detect it and reload the database automatically. 
The detection works also on network share and so permits that all user which have the same database open are not working on outdated databases.

This does not handle concurrent edition of the same .kdbx file. In this case a lock file or a merge mechanism is also necessary.