[Look and feel KCM] Fix copying color scheme data

Summary:
Using one argument with "KSharedConfig::openConfig(filename)" actually
results in using the default value for the second argument, "FullConfig".
Which results also in "Blend kdeglobals into the config object."
As a result the color scheme file opened in KCMLookandFeel::setColors(...)
this way has the complete global config groups and settings (with the data
as stored on the disc, not in any runtime working copies) mapped into the
profile (incl. e.g. the group "[General]" with the entry "ColorScheme").
So when then all the groups are copied over from the scheme config, actually
also all the stored global config groups and settings are copied over,
overwriting the not yet synced [General]/ColorScheme entry, which was just
set a few lines before, again with the old value from the storage.

Possibly the old code which was doing the sync right after setting the new
scheme value had been there exactly to protect against that
(cmp. change in cf49d415e7bb30e98c0e7529e7307d8449b8ffcd).

Reviewers: broulik, davidedmundson, mart

Reviewed By: broulik

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D10259