set min-width on tree-list buttons

avoids misaligned buttons when the content changes

IPython/html/static/style/style.min.css

@@ -8070,6 +8070,9 b' ul.breadcrumb span {'
 .item_buttons {
   line-height: 1em;
 }
+.item_buttons .btn {
+  min-width: 13ex;
+}
 .toolbar_info {
   height: 24px;
   line-height: 24px;

IPython/html/static/tree/less/tree.less

@@ -103,6 +103,9 b' ul.breadcrumb {'
 
 .item_buttons {
     line-height: 1em;
+    .btn {
+        min-width: 13ex;
+    }
 }
 
 .toolbar_info {