Add a separator class to renderer, rename previous separator to splitter and tweak the style

src/renderer/index.html

@@ -17,7 +17,7 @@
       <input id="uploaded-file" type="file" style="display:none">
       <button id="btn-upload-file">Load File</button>
       
-      <div class="separator"></div>
+      <div class="splitter"></div>
       
       <div> <!-- needed to stop the select from messing up -->
         <select id="opt-messages-per-page">

src/renderer/style.css

@@ -24,12 +24,16 @@ body {
   -ms-user-select: none;
 }
 
-#menu .separator {
+#menu .splitter {
   width: 1px;
-  margin: 9px 0;
+  margin: 9px 4px;
   background-color: #5D626B;
 }
 
+#menu .separator {
+  flex: 1 1 0;
+}
+
 #menu :disabled {
   background-color: #C2C2C2;
   cursor: default;