LibMyth: pass const QStrings by reference (cppcheck)
LibMythmetadata: pass const QStrings/MetadataLookupList by reference (cppcheck)
LibMythprotoserver: pass const QStringLists by reference (cppcheck)
LibMythtv: pass const QStrings by reference (cppcheck)
LibMythupnp: pass const QStrings by reference (cppcheck)
MythFrontend: pass const QStrings by reference (cppcheck)
LibMythbase: pass const QStrings by reference (cppcheck)

And bump the API version, plugins must be recompiled if the use the
changed methods.