Port to KPluginMetaData/KF6 no longer having a property metaDataFileName

((Stump branch forked in place of original commit, as other later changes conflict.))

External metadata no longer supported, thus the property gone.

Replaces the 2 commits "PluginController changes - why?" and follow-up "Adapt to changes in the plugin system - part 1":

• drops empty lines inserted
• drops unneeded include changes
• updates the commit message to reflect the only left change
• drops the change to the version metadata check, in first commit and the fix-up in the other:
  • original code relied on QVariant handling both int & string types and converting both to int value which then is compared to KDEVELOP_PLUGIN_VERSION numerical literal
  • new code only worked for number type of value, relying in fixed version on operator==(QJsonValue, QJsonValue) and implicit conversion from KDEVELOP_PLUGIN_VERSION numerical literal to QJsonValue
  • type could be string as in old config used with the plugin JSON metadata typically
  • type is number (double) though in case of KTextEditor plugins where the JSON metadata is manually generated in initKTextEditorIntegration()
  • no idea where plugins would come in for kf5 & kf6 not in versioned plugin path and with a string version value. But nothing that needs to be changed during porting.