fix(FCKeditors): Use unblockFCKeditors() on showFieldset() (refs #4399)

On Firefox, FCKeditors in non-visible tabs seems to be blocked and unusable.
So, on displaying the tab, the FCKeditors contained in the tab are "unblocked"
by collapsing/expanding their's toolbar.
Merge pull request #1052 from eguaj/3.1-A4399

fix(FCKeditors): Use unblockFCKeditors() on showFieldset() (closes #4399)