Steps
-Add a new character attribute (RES_CHRATR_HIGHLIGHT)
-Get this character attribute via SwFont class just like
background
-If has highlight, then paint that, otherwise paint background
-Extend UNO API
-Implement DOCX export and import filter
Note: By now character highlight can't be set via UI.
It's a next step to add a highlight option.

Change-Id: I7f81e173744bf256891487f898d06dbf372a2f88