HelloWorld: Add a simple "Hello World!" app showing the basics.

This also introduces a Demos/ directory where I hope to add cool things.

Demos/HelloWorld/.gitignore

@@ -0,0 +1,3 @@
+HelloWorld
+*.o
+*.d

Demos/HelloWorld/Makefile

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+include ../../Makefile.common
+
+OBJS = \
+    main.o
+
+APP = HelloWorld
+
+DEFINES += -DUSERLAND
+
+all: $(APP)
+
+$(APP): $(OBJS)
+	$(LD) -o $(APP) $(LDFLAGS) $(OBJS) -lgui -lcore -lc
+
+.cpp.o:
+	@echo "CXX $<"; $(CXX) $(CXXFLAGS) -o $@ -c $<
+
+-include $(OBJS:%.o=%.d)
+
+clean:
+	@echo "CLEAN"; rm -f $(APPS) $(OBJS) *.d
+

Demos/HelloWorld/main.cpp

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+#include <LibGUI/GApplication.h>
+#include <LibGUI/GWindow.h>
+#include <LibGUI/GWidget.h>
+#include <LibGUI/GLabel.h>
+#include <LibGUI/GButton.h>
+#include <LibGUI/GBoxLayout.h>
+
+int main(int argc, char** argv)
+{
+    GApplication app(argc, argv);
+
+    auto* window = new GWindow;
+    window->set_rect(100, 100, 240, 160);
+    window->set_title("Hello World!");
+
+    auto* main_widget = new GWidget;
+    window->set_main_widget(main_widget);
+    main_widget->set_fill_with_background_color(true);
+    main_widget->set_background_color(Color::White);
+    main_widget->set_layout(make<GBoxLayout>(Orientation::Vertical));
+    main_widget->layout()->set_margins({ 4, 4, 4, 4 });
+
+    auto* label = new GLabel(main_widget);
+    label->set_text("Hello World!");
+
+    auto* button = new GButton(main_widget);
+    button->set_caption("Good-bye");
+    button->set_size_policy(SizePolicy::Fill, SizePolicy::Fixed);
+    button->set_preferred_size({ 0, 20 });
+    button->on_click = [&] (GButton&) {
+        app.quit();
+    };
+
+    window->show();
+
+    return app.exec();
+}