Show panel-only options in preferences dialog only if panel plugin is actually loaded.

After clicking on the Clear button, bring the input focus back to the main entry.
Add keyboard shortcuts for file menu items.
Cleanup DictData struct, reorder elements and remove elements which are only used in the preferences dialog.
Add search method selection to the preferences dialog, including new mode to use the search method from the last time.
Add support for command line options for xfce4-dict command.

(Old svn revision: 4500)