Detect background changes to database file.

This gives the option to reload the database.

TODO:
 - Settings for reloadBehavior (ask, reloadUnchanged, ignore)
 - Improve notification, by using a header instead of dialog: nicer, less
intrusive, gives more options to user, and works better when multiple databases
are open.
 - Keep tab order on reload.
Auto-reload settings.
Fix currentMode() while DB is not yet open.
Fix StringField copy.
Fix entry partial matching rules.
Filter GetAllLogins entries, to return only entries with an URL.
Notifications pseudo-code.
Implement option to search all open databases.