Add separator between paned stacks in main window

A GtkSeparator was added between paned stacks in main window to align it with the separator in the headerbar. Since the sidebar already had a border, be need to remove it.

main/data/theme.css

@@ -74,6 +74,11 @@ window.dino-main .dino-sidebar > frame {
     border-bottom: 1px solid @borders;
 }
 
+window.dino-main list.sidebar {
+    border-left: 0px;
+    border-right: 0px;
+}
+
 .message-box {
     transition: background .05s ease;
 }

main/data/unified_main_content.ui

@@ -66,6 +66,15 @@
                 <property name="name">list-pane</property>
             </packing>
         </child>
+        <child>
+            <object class="GtkSeparator" id="paned_separator">
+                <property name="orientation">vertical</property>
+                <property name="visible">True</property>
+                <style>
+                    <class name="sidebar"/>
+                </style>
+            </object>
+        </child>
         <child>
             <object class="GtkOverlay">
                 <property name="visible">True</property>