Implement testing tab to match the MacOS front-end

[DavidKinder]

Following the OSX front-end, merge the skein and transcript into a new "Testing" tab. There are several related changes associated with this:

• Remove the "Replay" menu.
• Allow different selections in the two skein panels.
• Selected knots in the skein don't need to be ones playing.
• The skein is no longer horizontally centred in the tab if less wide than the tab.
• Reduce spacing between skein knots.
• "Test Me" knots have italic knots below them for the output they produce.
• Drag and drop of skein nodes with animation.
• Undo and redo of skein, change trim warning.

[DavidKinder]

Started work on this on the "testing_tab" branch.

[DavidKinder]

The main body of this work has now been merged to master.

[DavidKinder]

The latest MacOS front-end does not support labels/annotations in the Testing tab, except for the case of one knot, which can be marked as the "winning knot" (i.e. an annotation of "***"), like this:

[DavidKinder]

Commit b3745fa makes labels/annotations consistent with the MacOS front-end: labels are no longer directly available, except for the "Set Winning Command" menu option.

[DavidKinder]

Commit 7e5d3bc adds splitting up the output of "test me" commands into test sub-item nodes in the skein.

[DavidKinder]

Commit c092cef adds drag and drop to the skein.

[DavidKinder]

Closing this issue as the work is substantially complete. The last remaining item, undo and redo, has been moved to a separate issue: #61