the start offset in preedit string
the end offset in preedit string
Make a copy of the #WebKitInputMethodUnderline.
Free the #WebKitInputMethodUnderline.
Create a new #WebKitInputMethodUnderline for the given range in preedit string
the start offset in preedit string
the end offset in preedit string
Create a new #WebKitInputMethodUnderline for the given range in preedit string