Add GL2PS handler for vtkDataLabelMapper.

Change-Id: I2ca429146154606d6fe7950e6dc4397688781fff
Add note to make header tests happy.

Change-Id: I7159e0c8249d00aca6cce8fa734e072c8661537a
Merge topic 'selection-label-export' into master

78d211ed Add GL2PS handler for vtkDataLabelMapper.
Merge topic 'selection-label-export' into master

e040a636 Add note to make header tests happy.