configuration/config: Move config loading and saving to functions based off groups

Over time our config values have grown quite numerous in size.
Unfortunately it also makes the single functions we have for loading and
saving values more error prone.

For example, we were loading the core settings twice when they only
should have been loaded once. In another section, a variable was
shadowing another variable used to load settings from a completely
different section.

Finally, in one other case, there was an extraneous endGroup() call used
that didn't need to be done. This was essentially dead code and also a
bug waiting to happen.

This separates the section loading code into its own separate functions.
This keeps variables only visible to the code that actually needs it,
and makes it much easier to visually see the end of each individual
configuration group. It also makes it much easier to visually catch bugs
during code review.

While we're at it, this also uses QStringLiteral instead of raw string
literals, which both avoids constructing a lot of QString instances, but
also makes it much easier to disable implicit ASCII to QString and
vice-versa in the future via setting QT_NO_CAST_FROM_ASCII and
QT_NO_CAST_TO_ASCII as compilation flags.