Restore and extend centering of article list cell’s content

Note : to be visible, animation must be started after the attributed string is drawn
Follow up on issue #277 (folder name centering)

For centering content of cells in folder list, use the technique already used for centering content of cells in article list.

Note : we had to transfer font assignment to -outlineView:objectValueForTableColumn:byItem:
instead of -outlineView:willDisplayCell:forTableColumn:item:. Otherwise, drawing the attributed string by ourself did not work.
Merge pull request #293 from barijaona/bugfixes

Vertically center text in the folder list and in the article list