Non-DOM-modifying selection

Create a collapsed selection directly in handleLinearUpOrDownArrowKey, instead
of inserting, selecting and removing a $cursorHolder span. The original purpose
of the insertion technique seems to have been to give a non-collapsed selection
for the native action to work on, then delete the superfluous content
afterwards. However, since Ifd5000fc5e667cb9fd6db0e97cf8a03b4da45553 we have
been deleting the $cursorHolder *before* the native action, so the native
action acts on a collapsed selection in any case.

Also stop treating slugs specially, as there is now no text insertion to create
problems in them.

Change-Id: If507cc2bcc363c1d637409dfa191942c967a7c59
Merge "Non-DOM-modifying selection"