multimedia/opentoonz: massage dependencies, hand off maintainership

This is all rozhuk.im's work, (but without a name to attach I can't
easily set --author, so let's stick to an 'Obtained from' line).

• add installation shortcut to apps menu
• add missed deps: print/freetype2, multimedia/ffmpeg
• add WITH_DEBUG CXXFLAGS
• add pkg-message about missed tiff support
• drop libjson-c.so, gettext-runtime deps, it required only for libmypaint.so
• remove glib20 dep
• remove .gitkeep files
• switch to USES= blaslapack:openblas from LIB_DEPENDS= libgslcblas.so:math/gsl

PR: 256713
Approved by: tcberner (kde@)
Obtained from: rozhuk.im@gmail.com