make minor changes to the dialog to get it to work with the combobox, table and box layout

r=cmanske, a=jar

editor/ui/dialogs/content/EdImageProps.xul

@@ -180,6 +180,7 @@
             <select 
               id      = "widthUnitsSelect" 
               size    = "1"
+              style   = "width: 100px; min-width: 100px; max-width: 100px;"
               onclick = "doOverallEnabling()"/>
             <!-- option elements are appended by JS -->
           </td>
@@ -209,6 +210,7 @@
             <select 
               id      = "heightUnitsSelect" 
               size    = "1" 
+              style   = "width: 100px; min-width: 100px; max-width: 100px;"
               onclick = "doOverallEnabling()"/>
               <!-- option elements are appended by JS -->
           </td>