Introduced in 846d043e

Closes #779

 - Background parsing now never goes past the visible part of the
   document.

 - We only store style information for lines that are actually visible
   (and then keep it cached).

 - When the document changes, background highlighting always
   re-highlights from the change to the end of the visible part.

 - Gets rid of compareState mode methods and the hairy heuristic that
   tried to simulate it when absent.

 - The onHighlightComplete callback was removed, since it no longer
   really applies -- the document is only fully parsed when scrolled
   to its end.

This should help preserve memory (a huge document will no longer
immediately have parser state and highlighting information built up
for all lines, but only for the parts that you look at), and removes
the pathological case when you, for example, are typing at the top of
a huge XML document, and opening or closing a tag causes a whole
re-highlight to cascade all the way to the bottom because the
compareState will detect a change.

Issue #688

Fixes a problem where the editor would always be 30 pixels smaller
than its set height.

The height is now set on the outer wrapper DIV again, and some
height: 100% tricks are used to make height information flow
both ways (inside-out in demo/resize.html mode, outside-in in
normal, fixed-height mode).

Closes #775

Closes #773

The hack is horrible, but it seems to do the trick.

Also simplifies context-menu handling somewhat.

Issue #755

Some things appear to trigger it to scroll, with very bad
effects.

Closes #759

Closes #761

Closes #764

Fix bad rules in themes, measure width correctly.

Closes #765

It was passing undefined to setTimeout.

Closes #763

Closes #760 and fixes line backgrounds ending up behind editor background.

Discussion in issue #762

By clipping its top/bot to the borders of the outer div

The previous one broke line numbers

A *real* cleanup / reorder is becoming painfully needed, but this'll at
least make the tests pass.

Add CodeMirror.defineOption function for defining new options.

Add findMatchingBracket to the methods exported by this extension.

Closes #746

Prevents styled backgrounds from hiding the selection

It is now more structured (knows about ranges) and allows automatic
unfolding when the cursor enters the folded range.

This allows multiple handlers to listen for a single event, and makes
the API a bit more uniform.

Issue #725

Makes Hebrew text mostly usable

This will make fixedGutter work properly, and make it the only model
(no more gutter that scrolls out of view).

Also reorganizes the basic css file to be easier to modify for users.

Issue #730

Issue #294

Get rid of eolSpanContent mess.

Preparation for RTL text handling.

No more random {x,y,yBot} thrown into the mix.

Experimental for now.

- Multiple gutters are explicitly created with the gutters
  option
- Markers go into a specific gutter, new setGutterMarker,
  clearGutter methods
- The line number gutter is separate
- The 'fixedGutter' feature is removed (painful to do now,
  was shaky anyway)

Closes #738

When dragging there is no src attribute needed on the dummy image.