launcher/index.css: Align Landing folder and path

The LandingWidget-folder was not in vertical alignment with the
LandingWidget-path. This fixes the vertical alignment by setting
the :before which handles the baseline problem and also sets the
line-height to 1em to ensure the alignment is correct.

src/launcher/index.css

@@ -108,10 +108,14 @@
   background-origin: content-box;
   background-repeat: no-repeat;
   clear: none;
-  padding-bottom: 2px;
   padding-left: 12px;
-  height: 12px;
-  width: 12px;
+  height: 1em;
+  width: 1em;
+  line-height: 1em;
+}
+
+.jp-LauncherWidget-folder:empty:before {
+  content: "\00a0";
 }
 
 .jp-LauncherWidget-path {