Fixed alignment of admin menu icons

Tracker item: http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=30347

Joomla 3.1 has introduced the menu icons in the admin top nav again (for components, etc).
However, when using the standard 16x16px sized icons, the alignment is off.
This fix corrects that issue.

This fix also prevents the icon from showing up on narrow-screen (because there is no room for it and the menu item title overlaps the icon).

#### Testing instructions
If you don't have a component with an icon, add an icon to a component by placing the url (relative to the admin folder) of the icon in the 'img' field of the #__menu table for that component.

Now compare the menu item in the administrator menu (under Components).
Also check on narrow-screen (resize browser to less than 738px wide)