Plasma5 ports

At the moment KDE ports use bsd.kde4.mk to handle their dependencies. When
working on the ports for KDE Frameworks and Plasma5 it seemed to be more
reasonable to create a new kde.mk instead of adding an bsd.kde5.mk.

The kde.mk in this review is a stripped down version of the one we are using in
the KDE Test repositories plasma5 branch [1] to only contain the parts relevant
to the current KDE4 ports in the portstree [2].

Changes to the KDE Ports needed by this:

Replace USE_KDE4 by USE_KDE [3]
Add USES=kde:4 [4]
[1] http://src.mouf.net/area51/view/branches/plasma5/KDE/Mk/Uses/kde.mk
[2] The version in the plasma5 branch also handles frameworks/plasma5 and
    handles MASTER_SITES via a KDE_DIST variable similar to bsd.qt.mk for Qt
    Ports -- I chose to leave this out for now, as the diff is already large
    enough.
[3] I chose USE_KDE instead of USE_KDE4, USE_KDE5, USE_KDEX as the version we
    want is already specified as argument to kde:<arg>
[4] For KDE Frameworks and Plasma5 ports this would be kde:5

PR:             210667
Approved by:    portmgr, mat (mentor), rakuco (mentor)
Reviewed by:    mat, rakuco
Differential Revision:   https://reviews.freebsd.org/D6961