Maintenance options

[AlUlkesh]

I started putting maintenance options in their own new tab:

The Rebuild exif cache function is not new, but has been moved here now (for more details, see #44).

Update directory names in database is for when you have moved whole directories to a new location. Normally you would lose the exif and ranking information that was stored in the database. With this function you can restore it.

This commit also has some internal changes:

• All filenames are now stored and processed with their abspath
• Database gets upgraded to version 2 on the first run
• If a ranking is not found with the full pathname, another try is made with the filename
• ranking None reworked

[AlUlkesh]

Update due to #58

• Switched from abspath to realpath
• Database gets upgraded to version 3 on the first run