94 95 encrypt preferences thumbnails (also 77 a bit)

Closes #94, 
Closes #95,
#77 

### WARNING: this branch invalidates your current preferences files and thumbnails. Please uninstall application first before launching this branch and ensure you have made backups of the things you want to keep!!!

First commit:
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. 

Second commit:
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.

Third commit:
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