graphics/gmic-qt: Update to 3.5.0
Switch to the offical gmic tarball which has included everything we
need since version 2.2.2. This greatly simplifies the Makefile and
reduces download overhead. Plus, Sébastien Fourey, the author of
G'MIC-Qt sadly passed away in early October of last year, so we can
no longer use his GitHub repo as a source. Rest in peace.
Use Qt6 instead of Qt5.