[eclass] Add add_kdeapps_dep function.
[eclass] Add KDE Applications support to KDE 4 eclasses.

Handle SRC_URI appropriately, and automatically block the kde-base package with
the same name.

Adjust add_kdebase_dep to transparently handle the new situation - use the new
kde-apps category for all packages except kdelibs (which has no further 4.x
releases).

Todo, handle Plasma 5 and any other packages not part of KDE Applications.
[kde-apps/oxygen-icons] Migrate from kde-base.
[kde-apps/kcalc] Migrate from kde-base.