extraSelections is a local variable as well as a member
function of BaseTextEditorWidget, but if the full
qualified name is used the compiler thinks that this
is a static function call, which happens with the
MSVC 2010. Use the this pointer to use the correct
function.

Change-Id: I44ce96b2c17e7259274f2103ce70ae9b2ae3a56b
Reviewed-by: hjk <hjk121@nokiamail.com>