Display logical line numbers in DiffEditorPage

Display logical line numbers in DiffEditorPage

Give the DiffEditorPage a special LineNumberRulerColumn that can
display logical line numbers of the shown hunk lines, corresponding
to the line numbers in changed file versions.

The ruler menu has a toggle action to switch between this logical
display and the normal physical line number display.

* Store the line numbers in the DiffRegions produced by the
  DiffRegionFormatter. Add a CONTEXT DiffRegion.Type to be able to
  number context lines.
* Introduce a LogicalLineNumberProvider.
* Add a LogicalLineNumberRulerColumn that uses such a provider to
  determine line numbers for a diff side.
* Add a composite OldNewLogicalLineNumberRulerColumn that groups two
  such LogicalLineNumberRulerColumns (one for each side) together, and
  that contains the toggling logic for physical/logical line number
  display.

Change-Id: I53394854523d15187c21786ca54370561d84642d
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>