Adjust meta packages to allow co-existance with Plasma 5.

Have meta packages not install extra packages that come from Plasma 5,
avoiding conflicts.
Temporary fix until fixed upstream.  Deals with framewoks branch merged into master.
Unconflict kdebase-kioslaves from kio-extras.

Add a minimal use flag to kdebase-kioslaves, which allows kio-extras
to install at the expense of compiling software linking directly to
kdebase-kioslave software harder.  Note that no known software in portage
has this conflict.