Closes #3028

Issue #2995

Issue #2976

The code was confused beyond belief, and the hintWords
functionality will provide the same completion power
without loading an additional addon.

Allows convenient showing of panels (DOM nodes) above or
below an editor, automatically shrinking the editor to make
space.

Issue #2952

Implements a pluggable scrollbar model. Interface not documented yet,
but quite simple (see forthcoming simplescrollbars addon)

And manually compensate for our own scrollbar sizes.

This way, we're no longer dependent on the size of our scrollbars
being the same as the size of native scrollbars, and can start to
think about custom scrollbars.

a wiki engine

Also add support for "gutter" in add/removeLineClass.

Issue #2888

Issue #2908