Add a migration routine to EDataCalFactory.

Add a migration routine to EDataCalFactory.

Migration runs just before the main loop starts.
It's just a sequence of local directory renames.

~/.evolution/cache/calendar  -->  $XDG_CACHE_HOME/evolution/calendar
~/.evolution/cache/memos     -->  $XDG_CACHE_HOME/evolution/memos
~/.evolution/cache/tasks     -->  $XDG_CACHE_HOME/evolution/tasks
~/.evolution/calendar/local  -->  $XDG_DATA_HOME/evolution/calendar
~/.evolution/memos/local     -->  $XDG_DATA_HOME/evolution/memos
~/.evolution/tasks/local     -->  $XDG_DATA_HOME/evolution/tasks

We also migrate Evolution-Exchange account storage:

~/.evolution/exchange        --> $XDG_DATA_HOME/evolution/exchange

(After first cleaning up Exchange attachment cache directories.)