LineCursor can loop forward, but not back

[microbit-robert]

This is in normally navigation mode only. Move mode works as expected.

We've patched this temporarily. Note, this patch also contains changes to remove the stack connection cursor positions. Disregard changes to validLineNode and validInLineNode. Patch diff.

Summary of relevant changes:

• Change the prev and out methods to call getPreviousNode with the loop param set to true.
• I also pass the isValid function to getLastNode, which is passed through getNextNode where it's called inside getLastNode (required to avoid errors on looping back).