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

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

GIT_SILENT

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

No impact on the rest of the test, but I was wondering if it might
break anything, better restore it just in case.

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.

calling deleteGroup only deletes all entries but the group does still
exists in listGroups

this is somewhat irritating since calling exists on that group will
return false

with this patch the group does no longer exists in listGroup

"Policy CMP0115 is not set: Source file extensions must be explicit.  Run
  "cmake --help-policy CMP0115" for policy details.  Use the cmake_policy
  command to set the policy and suppress this warning.
"
We need to add file extension