add indexOf family of function to OUStringBuffer
add indexOf family of function to OUStringBuffer

it is fairly common when converting String to OUString
to find code that search for thing and replace/delete
these things... the later need OUStringBuffer, the former
OUString.
This add the search feature provided by the indexOf family
of function found in OUString, into OUStringBuffer

Change-Id: I2de818e6317a94ed520f338ebc9ed08d5f17765c

JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core