User definable athlete library location

Allow the user to select a directory to use for the
athlete libary. Leaving it blank will continue to use
the default location (dependent on platform).

If it is changed the user will be asked to confirm
and then GC shuts down.

Restart wouldn't work due to issues with the DBAccess
QSqlDatabase connection (which is really annoying). I
will look into this as a restart option would be much
cleaner.

Its only taken 4 years to implement this feature! (ouch).

Fixes #282.