Nheko 0.7.0

Since 0.7.0 should release any minute now and it was already tagged, these ebuilds should work for it (tested locally).

This also drops the CXX_STANDARD added in #40 (closed), because I don't know, why it would be needed. The nheko CMakeLists already sets the standard via a toolchain file. Also the correct C++ standard seems to be applied on my systems. If that is still an issue, I can of course add it back.