Base: Add the serenity-application HackStudio template

This is a template which instantiates into what you'd write to start out
a basic Serenity GUI application. It contains a CMakeLists.txt file
which describes what each declaration does, a simple GUI application
which uses layouts, widgets and callbacks, and comes with a minimal set
of pledges which the user can add to as necessary.

Base/res/devel/templates/serenity-application.ini

@@ -0,0 +1,5 @@
+[HackStudioTemplate]
+Name=SerenityOS GUI Application (C++)
+Description=Template for creating a GUI-based SerenityOS application with CMake.
+Priority=90
+IconName32x=serenity-application

Base/res/devel/templates/serenity-application.postcreate

@@ -0,0 +1,28 @@
+#!/bin/sh
+
+echo > $2/CMakeLists.txt <<-EOF
+# NOTE! Make sure to edit this file and remove the comments before submitting a
+# PR for a new application.
+
+# Defines your application component. If the application is essential to the
+# system's operation, add REQUIRED. If it would be beneficial for the user that
+# this application is built by default, add RECOMMENDED.
+serenity_component(
+    $1
+    TARGETS $1
+)
+
+# Place source files here. You should also add auto-generated headers, if you
+# have any.
+set(SOURCES
+    main.cpp
+)
+
+# Change this to something cool. :^)
+serenity_app($1 ICON filetype-executable)
+
+# You should place all the libraries that your application uses here. You can
+# identify each library by their include path. An exception is LibCore, which
+# is linked to all components by default.
+target_link_libraries($1 LibGUI LibGfx)
+EOF

Base/res/devel/templates/serenity-application/main.cpp

@@ -0,0 +1,40 @@
+#include <LibGUI/Application.h>
+#include <LibGUI/BoxLayout.h>
+#include <LibGUI/Button.h>
+#include <LibGUI/Frame.h>
+#include <LibGUI/MessageBox.h>
+#include <unistd.h>
+
+int main(int argc, char** argv)
+{
+    if (pledge("stdio recvfd sendfd rpath wpath cpath unix", nullptr) < 0) {
+        perror("pledge");
+        return 1;
+    }
+
+    auto app = GUI::Application::construct(argc, argv);
+
+    if (pledge("stdio recvfd sendfd rpath", nullptr) < 0) {
+        perror("pledge");
+        return 1;
+    }
+
+    auto window = GUI::Window::construct();
+    window->set_title("Form1");
+    window->resize(96, 44);
+    window->set_resizable(false);
+
+    auto& main_widget = window->set_main_widget<GUI::Widget>();
+    main_widget.set_fill_with_background_color(true);
+
+    auto& layout = main_widget.set_layout<GUI::VerticalBoxLayout>();
+    layout.set_margins({ 16, 16, 16, 16 });
+
+    auto& button = main_widget.add<GUI::Button>("Click me!");
+    button.on_click = [&](auto) {
+        GUI::MessageBox::show(window, "Hello friends!", ":^)");
+    };
+
+    window->show();
+    return app->exec();
+}