Selection timing flip

[jukibom]

Originally requested by @Zernys in #86

A shortcut to flip the timing order of a selected set of notes. Many symmetrical patterns can be made by first making the first half, either manually or through path builder, then copy-pasting, flipping it and overlapping the two ends. However, this currently requires every note to be manually reordered on the timeline.

[jukibom]

Oh, so flipping the position on the timeline instead of the canvas. interesting...