!!! WARNING: this invalidates your current preferences files. Please uninstall application first before launching this branch !!!

Moved the creation of the ConcealCrypto to the Utils class and let the service get it from there. This way the EncryptedPreferences method can get the same ConcealCrypto instance in order to encrypt/decrypt the preference files. Now the preferences are saved in their encrypted state. Next step: encrypting thumbnails.
!! WARNING: launching this branch will invalidate all your current thumbnails, please uninstall the app first !!

Thumbnails are now saved encrypted. Created the thumbnail manager, which handles all the thumbnail related operations. It can create them with encryption, and it can retrieve them with decryption.

TODO: recreate the thumbnails.
Now the thumbnails get cached as long as we are in the same folder. Also reworked the check for whether a file is locked, unlocked or being processed, so basically fixing a part of #77
Fixed problem that sometimes thumbnails did not show the correct image. Should be fixed now.
- now each time i set the thumbnail I set the view to invalid, to force the draw update
- now the retrieval and creation of thumbnails checks if this is already being done. If so, ignores the call in order to avoid unnecessary double calls.
Merge pull request #110 from AlexKolpa/94-95-encrypt-preferences-thumbnails

94 95 encrypt preferences thumbnails (also 77 a bit)