Implemented SkipMultiCursorBack as a counterpart to SkipMultiCursor

[masmu]

This PR adds SkipMultiCursorBack as a counterpart to SkipMultiCursor.

Since Undo does not reverse selections, you are somehow stuck if you have used SkipMultiCursor incorrectly one too many times.

The name for this new action is up for debate. Since I didn't want to change the name of SkipMultiCursor I could not think of something better than SkipMultiCursorBack.

In case changing the name of SkipMultiCursor is an option i would throw the following candidates into the ring:

• SkipNextMultiCursor and SkipPreviousMultiCursor
• SkipMultiCursorForward and SkipMultiCursorBackward

[dmaluka]

The name for this new action is up for debate. Since I didn't want to change the name of SkipMultiCursor I could not think of something better than SkipMultiCursorBack.

In case changing the name of SkipMultiCursor is an option i would throw the following candidates into the ring:

• SkipNextMultiCursor and SkipPreviousMultiCursor
• SkipMultiCursorForward and SkipMultiCursorBackward

To me SkipMultiCursorBack doesn't seem a very bad name. After all, what these 2 actions do is skip the current multicursor, not the next or previous one. (But I'd also welcome any better name suggestions.)

[JoeKar]

Looks OK to me too 👍.