Keychain support

As in old ticket: http://trac.kekaosx.com/ticket/30

- [ ] Support multiple passwords stored in Keychain
- [ ] Suggest them in the password field, compression and extraction (@Holger-Issle #468)
- [x] Option to try the stored passwords in extraction (not very efficient?) (@Holger-Issle #468)
- [x] Add a default password option in preferences (@own2pwn #118)
- [x] Add context menu support for encryption with default passwords
- [x] Add context menu support for encryption with custom passwords
- [ ] Remember password used for files using a fingerprint (i.e. MD5 or SHA256) (@MaxPower85)
- [ ] Add a list in plain text of used passwords (and be able to costumize it) (@finalcreator #1048)