Return stringlist/boolean types in KPluginMetaData::value

Plasma code is littered with
KPluginMetaData::value("someStringList").contains("blah")

This used to accidentally work fine because we would be converting
.desktop files and have no concept of what KPluginMetaData entry should
have been a stringlist or a string.

Since 49bc26a135bf92bcddecabfdd4f3f71f65540104 we now correctly work out
the service type of a .desktop file, and so when we parse it, entries
that should have been lists are now considered a QJsonValue of type
Array rather than String, meaning KPluginMetaData::value returns nothing
instead of the original comma separated list in the .desktop file.

Even though we should fix all Plasma isntances to use the correct
KPluginMetaData method for the data they're fetching, we can't afford
old installs to break.

REVIEW: 129770