Move spells_db into ProjectContext

It'll be exposed as turbo_batman, and contain all the context
properties. This way, we don't clutter the namespace, and have one place
to look for all the context properties.

Note that sizes didn't go into this, because:
a) It's used in too many places
b) It isn't really project specific - more to do with layout/ui

Resolves #5
Create FilterProxyModel

It is a simplified QSortFilterProxyModel that is specialized to
search for a role named "name", in a way that makes sense for
spells.
Add filter bar to SpellsPane

Using the funky new FilterProxyModel.

Resolves #4