Implement a clipboard, and copy and paste from a selection.

Who knew copy and paste was so complicated? Copying is complicated because the structure isn't flat, i.e. a block has references to tracks and rulers. Pasting is complicated because the clipboard events have to be shifted and clipped according to the destination selection.

Also, the tracks are typed in that it's not appropriate to paste control events into a note track. However, I'm going to ignore that and assume the user won't paste them if he didn't mean it.

Instead of having a special case clipboard, the clipboard is implemented as a normal block and tracks (rulers are not copied), in a clipboard namespace. That way, you can have multiple clipboards by copying them to different clipboard namespaces, edit clipboards in place, and the paste code is the same as the code that merges another project from disk.

Further ideas:

• use two selections and a "swap" command
• mouse chording for copy paste
• different mouse buttons are hard to do on the mac, so use standard for now
• merge with function... I think I can just do it at the REPL

More complicated pastes should be implemented as derivers, which are more flexible than editing operations. However, there could be a "derive in place" cmd to flatten deriver structure.