Login Dialog

[nurupo]

Make a login dialog similar to #12

For existing accounts, usernames should be pulled out of user data files (literally go through files and read unencrypted username and/or public id), password is used to call a decrypt function, and on successful decryption this should initialize Core instance and switch to the main window.

For new users, username is just any string no longer than TOX_MAX_NAME_LENGTH bytes (note that a UTF-8 character takes 1-4 bytes), and password is a string that will be used for encrypting the user data file.

Also, in addition to what is shown on the mockup, an option for import of a user data file should be added.

No encryption should be implemented here, the encryption is part of #66

A dummy GUI can be made now, but in order to function properly #66 needs to be completed.