Redo instead of toggling Z pane in text fields (except search bar) on OS X

Previously Cmd-Shift-Z worked for redo in text areas (Abstract, Extra)
but would toggle the Zotero pane in regular text fields. This change
allows Redo to work as expected.

The search bar gets focus when you first open the Zotero pane, though,
so we don't redo there, since it would be annoying if you couldn't close
the pane immediately with the same shortcut.