Copy/Merge a selection of PatternClips into a single new pattern

[regulus79]

This PR allows the user to select a group of PatternClips on the same track, rightclick, and select the option to "Copy to New Pattern Track", which will merge the selected clips into one new clip on a new PattenTrack.

Currently, this is only implemented for MidiClips, due to the limitation of PatternClips that they cannot store more than one AutomationClip or SampleClip in a row.