[Update] Combine in one window. Rename Main/Config Window to Main/Config Page accordingly.

For now, I put Config Page to below the main page temporary. Open Config Button does not work.

src-ui/app/main_page/main_section/MainSection.module.scss

@@ -0,0 +1,13 @@
+.container {
+    position: relative;
+    width: 100%;
+    height: 100%;
+    display: flex;
+    flex-direction: column;
+    justify-content: space-between;
+}
+
+.language_selector_container {
+    width: 100%;
+    height: 100%;
+}