build:2.6.0.v20100311

steps to reproduce:
1. insert a label, define hyperlink and hyperlink tooltip
2. preview in word/excel/ppt

expected result:
tooltip works, office software has screentip property in hyperlink, we should set tooltip in this property

current behavior:
doc works.
excel display tooltip as a comment.
ppt display no tooltip
fixed the problem.
verified in 2.6.0.v20100408-0630
verified in 2.5.3.v20100518-0830