New Features: Separate Paste into Add, Insert and Overwrite

[tin2tin]

Add: Places the pasted strip selection in first channel with room for the pasted strip at playhead position.

Insert: Places the strip in the channel with the active strip at playhead position. If there already is a strip at that position, a cut is made in the existing strip and the rest is pushed to the right, in order to make room for the new strip. This function should be disabled if more than one strip is to be pasted.

Overwrite: Places the strip in the channel with the active strip at playhead position. If there already is a strip at that position, the new strip overwrites the old one. This function should be disabled if more than one strip is to be pasted.