Added a vertically centered NSTextFieldCell subclass.

Class inspired by http://www.red-sweater.com/blog/148/what-a-difference-a-cell-makes and http://stackoverflow.com/questions/1235219/is-there-a-right-way-to-have-nstextfieldcell-draw-vertically-centered-text
Use vertically centered text cell for the progress cell.

Related to issue #174
Use also vertically centered cells for other columns of article list.

Issue #174