kcmshell6: Use KShell to split values coming from args option

The name suggests plural, but we'd handle the value.
In kde-cli-tools kcmshell5, we would do
"parser.value(QStringLiteral("args")).split(QRegularExpression(QStringLiteral("
+")))".

Test Plan:
Add debug output and expect all args to be part of args
./bin/kcmshell6 kcm_plasmasearch --args "bla1 bla2 bla3 'testme testmetoo'"