At the moment, we could say that plasma is co-installable by chance,
it's only working because KF5 dropped the apps relative directory.
This change introduces a PLASMA_RELATIVE_DATA_INSTALL_DIR that will
be available to know where in share the components are and
PLASMA_DATA_INSTALL_DIR to know where data has to be installed to.
Reviewed by David Edmundson
CCMAIL: plasma-devel@kde.org