Add a new port multimeda/gstreamer1-qt5 (as slave of multimedia/gstreamer1-qt4)

The port for multimedia/gstreamer1-qt5 has existed in area51 for a while now,
and has been worked on by rakuco@ and myself.
It used the same distfile as multimedia/gstreamer1-qt4, but requires an
additional patch over the qt4-based one to fix compilation in C++11 mode.

To bring this port now into the official ports-tree the following was done:

• Add multimedia/gstreamer1-qt5 (obvously) as as a slaveport
• Replace the two patches in multimedia/gstreamer1-qt4 obtained from OpenBSD by the git commit from upstream (we already had it like that in the qt5 one)
• Import the C++11 (not strictly necessary for the qt4 one, but doesn't/shouldn't hurt)
• Add proxy deps to make stage-qa happy

Reviewed by: rakuco
Approved by: rakuco (mentor)
Differential Revision: https://reviews.freebsd.org/D7907