1st attempt @ unselectInto; cursor does anticursor

behavior change: keyboard select (Shift-Left/Shift-Right) will unselect
back into nodes that were selected out of, hence, call unselectInto on
nodes that were selectOutOf-ed, which are detected by checking if the
anticursor is inside the node.

Since there's no levelling up the cursor/anticursor pair, the node now
doesn't even touch the anticursor, the cursor handles that.

Selection in TextBlocks is extra broken.
:s/\<self\>/cursor/g in Cursor::selectDir #cosmetic

because it looks too much like seln
Merge branch 'dev' into feature.unselect-back-into
Merge pull request #144 from mathquill/feature.unselect-back-into

Feature: unselect back into nodes that were selected out of

#179 is a known issue