Silence some warnings when reading plugins metadata
It fixes such warnings as:
kf.coreaddons: Expected JSON property "NoDisplay" to be a single string. but it is a bool
On plasmashell start.
KPluginMetaData::value
expects the json value to be a string and prints a warning when it is not a string.
Using rawData().value instead also to use directly the Json parser and avoid this warning.
It is used that already very commonly.
It also saves a redundant string comparison.
Edited by Méven Car