ModelGallery: Add the new Model Gallery application :^)

This is an application analogous to WidgetGallery, in that it tests
various capabilities of LibGUI models. Right now it is pretty bare, but
as more work towards LibGUI models is done regarding persistent model
indices, more demos will be added.

Userland/Demos/ModelGallery/BasicModelTab.gml

@@ -0,0 +1,44 @@
+@GUI::Widget {
+    name: "basic_model_tab"
+    layout: @GUI::VerticalBoxLayout {
+        margins: [4]
+    }
+
+    @GUI::Label {
+        text: "Here is a basic model, displayed on a table widget. Its clients are updated via granular updates. You can add or remove items with the widgets below."
+        text_alignment: "CenterLeft"
+
+        thickness: 2
+        fixed_height: 34
+    }
+
+    @GUI::TableView {
+        name: "model_table"
+    }
+
+    @GUI::Widget {
+        layout: @GUI::HorizontalBoxLayout {
+        }
+
+        fixed_height: 30
+
+        @GUI::TextBox {
+            name: "new_item_name"
+            placeholder: "Enter some text to be added..."
+        }
+
+        @GUI::Button {
+            name: "add_new_item"
+            fixed_width: 22
+            fixed_height: 22
+            tooltip: "Add the text as an item to the model"
+        }
+
+        @GUI::Button {
+            name: "remove_selected_item"
+            fixed_width: 22
+            fixed_height: 22
+            tooltip: "Remove the selected item from the model"
+        }
+    }
+}