Support placing cursor in ligature with mouse or touch

Support placing cursor in ligature with mouse or touch

We need to find out the closest element in the ligature to
the point we clicked (or tapped), currently we do this by
dividing the width of that ligature glyph evenly by the number
of characters it covered. We only support Common and Greek script
at this point, ligatures in other scripts are still handled as a
whole.

Task-number: QTBUG-19260
Reviewed-by: Eskil
(cherry picked from commit 5338d78aa9d80ddd2bcb21e6b22cd2cf1522a7d3)

Change-Id: Ic747e9458d561aca0848dcd1e8b94e0a23fd8189
Reviewed-on: http://codereview.qt.nokia.com/196
Reviewed-by: Jiang Jiang <jiang.jiang@nokia.com>