optimize text split methods

With this change, the time consuming string width calculation is called
as late as possible.
The split methods have been changed to reuse the current instance for
the remaining part of a pair.

Change-Id: Idc68e12432fe2fd529438f7b575e8c561f3294e7
Signed-off-by: Carsten Hiesserich <carsten.hie@gmail.com>
20 files changed