The code was using a QSet (hash-based), use a std::set instead
for unicity, it gives us sorting for free.

We probably want to do the same in groupList(), but that's separate.

This is the case where we expected to see Key[$d] in the config file,
and it was somehow broken. When saving, the key was omitted, so when
reloading the kdeglobals key was present again.

Detected when trying to write a unittest for a different patch...

I had to reshuffle the unittest a bit because testThreads calls
testSimple which didn't expect that the "[AAA]" group would actually
be deleted now.

GIT_SILENT

We are only interested in whether the file doesn't exist when it fails
to open. This saves a stat on every successful config file parse.

At the moment we are taking a bit of a detour by converting to char*.
Not a bit deal but reads better and ends up being less calls.

Whenever we read a config file, we are first parsing kdeglobalsrc. This
caches the parsed entries so that they can be reused as long as it
hasn't changed.

This reduces the parseConfig calls (which is mostly prominent at
early process startup) from 18 to 12 (33%) ksmserver-logout-greeter.
This also means there's better cache locality as well as less memory
allocated as a lot of objects are shared but I have not measured this.

NO_CHANGELOG

This reverts commit b3dc879e8b108c26c929bfbe551bcdf77f140e94.

This change breaks plasmashell startup and possible other apps as well.
Reverting so the root cause can be investigated without time pressure.