Add pulsing effect for placeholder items

src/App.css

@@ -63,6 +63,10 @@ body {
   background-color: #0056b3;
 }
 
+.PlaceholderItem {
+  animation: loading 1.5s infinite;
+}
+
 .PlaceholderItem > button,
 .ParentDirectoryItem > button {
 	background-color: #4D4F5D;
@@ -73,6 +77,15 @@ body {
 }
 
 .PlaceholderItem > button {
-  height: calc(16px + 10px * 2);
-  width: 6em;
+  height: calc(18px + 10px * 2);
+  min-width: 4em;
+}
+
+@keyframes loading {
+  0%, 100% {
+    filter: brightness(80%);
+  }
+  50% {
+    filter: brightness(100%);
+  }
 }