Fix bug 48859: make the tree columns in the XPI install and status dialogs resizable. r=jag a=brendan

xpinstall/res/content/institems.xul

@@ -58,6 +58,7 @@ Contributor(s):
                 <tree id="modulesTree">
                     <treecolgroup>
                       <treecol flex="1"/>
+                      <splitter class="tree-splitter"/>
                       <treecol flex="1"/>
                     </treecolgroup>
 

xpinstall/res/content/xpistatus.xul

@@ -60,6 +60,7 @@
                 <tree id="modulesTree">
                     <treecolgroup>
                       <treecol flex="1"/>
+                      <splitter class="tree-splitter"/>
                       <treecol flex="1"/>
                     </treecolgroup>
                     <treehead>