added initial window with grid layout and css styling

.gitignore

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+build

Makefile

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+main: main.cpp
+	g++ main.cpp -o build/main `pkg-config --cflags --libs gtkmm-4.0`

main.cpp

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+#include <iostream>
+#include <gtkmm/button.h>
+#include <gtkmm.h>
+#include <gtkmm/application.h>
+
+
+class MyWindow : public Gtk::Window
+{
+    public:
+        MyWindow(){
+            loadCss();
+            m_button = Gtk::Button("test");
+            m_button.signal_clicked().connect(sigc::mem_fun(*this,&MyWindow::onButtonClicked));
+
+            m_label = Gtk::Label("Hello!");
+            m_label.get_style_context()->add_provider(css_provider,GTK_STYLE_PROVIDER_PRIORITY_USER+1);
+            m_label.add_css_class("label");
+
+            m_picture = Gtk::Picture("/usr/share/doc/gtkmm-4.0/reference/html/image1.png");
+            m_picture.set_size_request(200,200);
+
+            m_layout_grid = Gtk::Grid();
+            m_layout_grid.get_style_context()->add_provider(css_provider,GTK_STYLE_PROVIDER_PRIORITY_USER+1);
+            m_layout_grid.attach(m_button, 0, 0);
+            m_layout_grid.attach(m_label, 1, 0);
+            m_layout_grid.attach(m_picture, 0, 1);
+            m_layout_grid.add_css_class("grid");
+
+            set_child(m_layout_grid);
+            set_size_request(800,600);
+        }
+        void onButtonClicked(){
+            std::cout<<"hello world";
+            std::flush(std::cout);
+        }
+        void loadCss(){
+            css_provider = Gtk::CssProvider::create();
+            css_provider->load_from_path("main.css");
+        }
+        Gtk::Picture m_picture;
+        Gtk::Button m_button;
+        Gtk::Label m_label;
+        Gtk::Grid m_layout_grid;
+        Glib::RefPtr<Gtk::CssProvider> css_provider;
+};
+
+int main(int argc, char **argv)
+{
+    auto app = Gtk::Application::create("org.gtkmm.example");
+    return app->make_window_and_run<MyWindow>(argc, argv);
+}

main.css

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+.label{
+    font-weight: 700;
+    font-size: 20px;
+    border: 2px solid red;
+}
+.grid{
+    background-color: yellow;
+}