Update devel/qbs to 1.6.0 and devel/qtcreator to 4.1.0

[tcberner]

Summary

• Update devel/qbs to 1.6.0
• Update devel/qtcreator to 4.1.0
• Add the QMAKE_LIBDIR_FLAGS hack we use in bsd.qt.mk for the Qt-ports. This should hopefully make it possible to upgrade qbs & qtcreator without deinstalling them first.
• As there is a qtcreator plugin linking against libqbscore.so, change the RUN_- and BUILD_DEPENDS into a LIB_DEPENDS.

Test Plan

Tested in poudriere: 9.3 (32&64), 10.3 (32&64), 12 (64)