Removing nesting indicator

It's a bit annoying when you hover over the tags, is only useful for
HTML/XML etc and ICEcoder covers many langs now plus clutters the
display a bit. Not so useful anymore.
Remove getNestLocationSub function
Removed nestDisplay class
Removed nestDisplay div
Nest display functions gone & code assist on diff

Lots of items removed as part of dropping this:

dontUpdateNest
dontSelect
htmlTagArray
startPosLine
startPosCh
cursorOrigCh
cursorOrigLine
nestDisplay
tagString

highlightBlock()
getNestLocation()
getNestLocationSub()
setPosition()

images/nest-tag-bg.gif

codeAssistToggle now working on diff pane also