Big bug hunt; saving/loading now work as expected
Moved and simplified the Profile class

This commit moves the Profile class out of Core, and simplifies its
functions.

Having the Profile class encapsulated within Core was visibly becoming a
poor decision - it was going to create problems in the longer run with
the login dialog. As such, this commit moves it to its own top-level
file. Profile has also been simplified to work just with the Tox
Messenger save data, and no longer with actual Tox instances & the
tox_load/save() functions.