Add StickyColumnPolicy enum.
Refactor updateStickyColumn and setPosition methods

Usage of the StickyColumnPolicy enum allows greater expression of what
we intend to do with the sticky column when the position of the caret is
changed.
Let setPosition set stick to EOL flag.
Let LineEndMotion set stick to EOL flag.
LineStartMotion allways resets stick to EOL flag.
Let '+' and '-' reset stick to EOL flag.
Unbind and del StickToEOLCommand / LineStartCommand

These commands are made obsolete by the changes in setPosition(). The
motions themselves will state whether the EOL flag should be reset,
hence working in any CommandBasedMode.
Remove unused 'stickToEOL' / 'stickToBOL' methods.

All of this is now handled by setPosition.
Merge pull request #296 from albertdev/stickycolumn-policy

Implement new Stickycolumn policy