Furnace v0.6pre15 (beta) - The Freeze

once again

changes

• preparation work for Flatpak release (thanks Colin Kinloch)
• honor XDG_CONFIG_HOME
• add sample preview volume setting as default was too loud
• fix OPM/OPZ wrong pitch range
• documentation updates - also docs are CC BY 3.0-licensed now
• drop usage of std::deque and std::queue
• OPL: fix drum muting
• fix X68000 preset