Add Watched Directories Tab.

Add Watched Directories Tab.

The tab allows editing of watched directories. A synthetic primary key was
added to the watched_directory table to allow use with QtSQL classes.
Watched directories can now be disabled. Triggers were added to automatically
update watched directories when the table contents changes.

Database::getWatchedDirectories can now return all or only currently enabled
directories. The latter is the default.

This change requires a schema update.