Add graphical interface

[Krzyhau]

Would be nice if CLI wasn't the only way to interact with Repacker, and some very simple GUI would be appreciated.

• establish the GUI library we'd use (I guess cross-platform would be nice, so that's a priority)
• figure out basic UX and layout
• implement

[Krzyhau]

I have decided to go for WPF as the GUI library of choice. Time will tell if that's gonna unleash the screams of non-Windows users.
In terms of design, I am inspired by VPKEdit and most likely it'll be similar in terms of functionality (as in both cases the software is used to preview and edit packages filled with proprietary file formats).

In the following days I'm planning to keep working on the interface on feat/gui branch.