The header classes for specific levels of headers, which should set the size of each header did not match the classes in the editor. This commit fixes that.

Issue #2085

Closes #2080

Add getHelpers and registerGlobalHelper, allow modes to specify arrays
of helper names, support modeProps in mode specifications.

Update foldcode and show-hint to make use of more powerful helper
system by defining an `auto` helper that tries multiple helpers, and
using that as default.

Issue #2025

Issue #1825

Demo was incorrectly named as showing 'Highlighter', fixed to 'Selection'

Issue #1765

Provides new option (highlightDifferences) and method highlightDifferences(bool)
to the MergeView plugin.

If true (or undefined) the original behaviour is maintained, which is to show
the differences (line highlighting and gutter graphics), if false then
the changes will not be shown.

Calling the method causes the visual change to occur 'immediately'

Signed-off-by: ciaranj <ciaranj@gmail.com>

Issue #1739