Allow KHelpCenter from Plasma 5.0 to handle KDE4 KHelpCenter duties

KHelpCenter from Plasma 5.0 can handle KDE4 duties.  Add a kde4_compat flag
that installs a desktop file to allow KDE4 applications to launch it.
Allow kdelibs to use an appropriately setup khelpcenter:5

khelpcenter:5 with the kde4_compat flag can work as the khelpcenter:4
package.  Allow kdelibs to use this.
Loosen dependencies on plasma-workspace to plasma-runtime.

Some packages depended upon plasma-workspaces, when they just needed
the runtime components.  The runtime is co-installable with Plasma 5.0,
while the workspace isn't.
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.