QuickShow: Allow panning and zooming the image instead of stretching it.

This needs more work and polish, but it's a step in a more pleasant and
useful direction.

Also turn QuickShow into a fully-fledged "application". (By that, I really
just mean giving it its own Applications/ subdirectory.)

Applications/QuickShow/main.cpp

@@ -0,0 +1,74 @@
+#include "QSWidget.h"
+#include <LibGUI/GAction.h>
+#include <LibGUI/GApplication.h>
+#include <LibGUI/GBoxLayout.h>
+#include <LibGUI/GLabel.h>
+#include <LibGUI/GMenu.h>
+#include <LibGUI/GMenuBar.h>
+#include <LibGUI/GWindow.h>
+#include <SharedGraphics/PNGLoader.h>
+#include <stdio.h>
+
+int main(int argc, char** argv)
+{
+    GApplication app(argc, argv);
+
+    auto menubar = make<GMenuBar>();
+
+    auto app_menu = make<GMenu>("QuickShow");
+    app_menu->add_action(GAction::create("Quit", { Mod_Alt, Key_F4 }, [](const GAction&) {
+        GApplication::the().quit(0);
+        return;
+    }));
+    menubar->add_menu(move(app_menu));
+
+    auto file_menu = make<GMenu>("File");
+    menubar->add_menu(move(file_menu));
+
+    auto help_menu = make<GMenu>("Help");
+    help_menu->add_action(GAction::create("About", [](const GAction&) {
+        dbgprintf("FIXME: Implement Help/About\n");
+    }));
+    menubar->add_menu(move(help_menu));
+
+    app.set_menubar(move(menubar));
+
+#if 0
+    if (argc != 2) {
+        printf("usage: qs <image-file>\n");
+        return 0;
+    }
+#endif
+
+    const char* path = "/res/wallpapers/sunset-retro.png";
+    if (argc > 1)
+        path = argv[1];
+
+    auto bitmap = load_png(path);
+    if (!bitmap) {
+        fprintf(stderr, "Failed to load %s\n", path);
+        return 1;
+    }
+
+    auto* window = new GWindow;
+
+    auto update_window_title = [&](int scale) {
+        window->set_title(String::format("QuickShow: %s %s %d%%", path, bitmap->size().to_string().characters(), scale));
+    };
+
+    window->set_double_buffering_enabled(false);
+    update_window_title(100);
+    window->set_rect(200, 200, bitmap->width(), bitmap->height());
+
+    auto* widget = new QSWidget(nullptr);
+    widget->on_scale_change = [&](int scale) {
+        update_window_title(scale);
+    };
+    widget->set_bitmap(*bitmap);
+    window->set_main_widget(widget);
+
+    window->set_should_exit_event_loop_on_close(true);
+    window->show();
+
+    return app.exec();
+}