#4 Drag and Drop exported note patterns into IntrumentTrack's

This makes it possible to drag exported note patterns (.xpt, .xptz) from the side-bar (file-browser) into a InstrumentTrack, it will be dropped as a new TCO. It works in both Song-Editor and BB-Editor. GitHub PR: https://github.com/LMMS/lmms/pull/5963 Note: this will not be merged directly into this fork!