Now that KF requires Qt 5.15.2; this basically reverts commit 2e8742e64fc0
with some trivial changes.

NO_CHANGELOG

clang-tidy checks:
readability-isolate-declaration and readability-braces-around-statements

KF task: https://phabricator.kde.org/T14729

GIT_SILENT

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.