add confirmation dialog to leave guild

dialogs/confirm.cpp

@@ -0,0 +1,30 @@
+#include "confirm.hpp"
+
+ConfirmDialog::ConfirmDialog(Gtk::Window &parent)
+    : Gtk::Dialog("Confirm", parent, true)
+    , m_layout(Gtk::ORIENTATION_VERTICAL)
+    , m_bbox(Gtk::ORIENTATION_HORIZONTAL)
+    , m_ok("OK")
+    , m_cancel("Cancel") {
+    set_default_size(300, 50);
+
+    m_label.set_text("Are you sure?");
+
+    m_ok.signal_clicked().connect([&]() {
+        response(Gtk::RESPONSE_OK);
+    });
+
+    m_cancel.signal_clicked().connect([&]() {
+        response(Gtk::RESPONSE_CANCEL);
+    });
+
+    m_bbox.pack_start(m_ok, Gtk::PACK_SHRINK);
+    m_bbox.pack_start(m_cancel, Gtk::PACK_SHRINK);
+    m_bbox.set_layout(Gtk::BUTTONBOX_END);
+
+    m_layout.add(m_label);
+    m_layout.add(m_bbox);
+    get_content_area()->add(m_layout);
+
+    show_all_children();
+}