Change Database class initialisation process

[Eitot]

This is a followup to #2083.

I believe it is not a good idea to call all the database setup code within the dispatch_once block of the singleton initialiser. I have seen several instances where an exception is thrown or the app will just crash if objects that are meant to be accessed on the main thread are accessed from within that block.

In anticipation of #2063 I investigated which database code is accessed during the loading process of the view hierarchy and found that only the arrayOfFields/fieldByName accessors are read, which don't require the SQLite database to be loaded. I therefore reduced the Database initialiser to just initialise the class and ivars and moved the database setup code into a separate ‑loadDatabaseStore method that is called from ‑applicationDidFinishLaunching:. I made arrayOfFields/​fieldByName lazily loaded and simplified the Field initialisation code by adding a convenience initialiser.