Add initial splitview option for filemanager

src/theme.scss

@@ -1827,6 +1827,27 @@ tag-input {
 // ----------------------------
 
 .filemanager {
+    .layout-root {
+        height: calc(100% - 30px);
+    }
+
+    .filemanager-layout {
+    }
+
+    .file-trees {
+        display: flex;
+    }
+
+    .spacer {
+        width: 2px;
+        background-color: #d3d3d3;
+        margin: 0 10px;
+    }
+
+    filetree {
+        // flex-basis: 50%;
+        width: 100%;
+    }
 
     .toolbar {
         display: flex;