Use existing AES cipher from libkeepass

Don't maintain an own implementation.
Merge pull request #10 from bhedrich/use-existing-crypto-functions-from-libkeepass

Use existing AES cipher from libkeepass
Add utf-8 header to source files
Code cosmetics
Add implementation of a Singleton pattern
Improve Conf class

Inherit from "Singleton" and add more functionality to it.
Merge pull request #13 from bhedrich/refacor-conf-class

Refacor conf class
Add gui support

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