Only invoke config changed callbacks from Config::Save, not
Layer::Save. The latter results in callbacks being called
once per layer, up to 7 times per save.
Currently, a simple typo in the system name will trigger an assert
message that complains about a "programming error". This is not
user friendly and misleading.
So this changes GetSystemFromName to return an std::optional, which
allows for callers to check whether the system exists and handle
failures better.
Originally, Layer contained a std::map of Sections, which containted a std::map
containing the (key, value) pairs. Here we flattern this structure so that only
one std::map is required, reducing the number of indirections required and
vastly simplifying the code.
Settings that come from the SYSCONF are now included in Dolphin's
config system as part of the base layer. They are handled in a
special way compared to other settings to make sure they are only
loaded from and saved to the SYSCONF (to avoid different, possibly
contradicting sources of truth).
The Config::AddLoadLayer functions call Load on the layer
explicitly, but Load is already called in the constructor,
so they'd cause the loader's Load function to be called twice,
which is potentially expensive considering we have to read an INI
from the host filesystem.
This commit removes the Config::AddLoadLayer functions because
they don't appear to be necessary.
