Character API improvements

This pull requests does the following:
* Adds two columns to characters table in DB
  * ```defaultChar``` is the character that is associated with the API and the pyfa character
  * ```chars``` is a text field which, in json format, stores the list of characters accessible by the api id/key pair
* Since the character from the API is now stored in the DB, automatic API updates can happen
* Added API Update button next to character selection on main screen (I tried to find a way to add it to a context menu for the skillbook image, but I am not too familiar with python / the underlying code)(also, I used a random image that somewhat relayed the meaning)

I think the changes made could help streamline the character API update process.

Please note I do not know how this will affect previous installations due to the DB schema change - I used a clean DB for my edits.

§ Sable Blitzmann