TextEditor: Don't launch a WebContent process until it's needed

The HTML and Markdown preview modes both use an OutOfProcessWebView to
render the preview pane, and we were instantiating this view from GML.
This caused us to always spawn a WebContent process alongside every
TextEditor instance.

Fix this by deferring the OOPWV construction until we actually need it.
This makes launching TextEditor on a text file quite a bit faster. :^)

Userland/Applications/TextEditor/TextEditorWindow.gml

@@ -19,9 +19,12 @@
             name: "editor"
         }
 
-        @Web::OutOfProcessWebView {
-            name: "webview"
+        @GUI::Widget {
+            name: "web_view_container"
             visible: false
+
+            layout: @GUI::VerticalBoxLayout {
+            }
         }
     }