Give Effector a version and a changelog, shown from the menu

The version is the project() line of CMakeLists.txt and nowhere else:
CMake writes it, with the text of CHANGELOG.md, into a generated
core/version.hpp, so the number and the release notes the binary shows
are the ones it was built from. effector --version (-V) prints it, and
the header-bar menu gains a section with Changelog, a window laying the
Markdown out as headings and bullets, and About Effector.

The core tests hold the newest changelog entry to the current version,
so a bump cannot go in without its notes. Releases are tagged vX.Y.Z;
this is 0.5.0, and the changelog reconstructs 0.1.0-0.4.0 from the
history.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/gui/window.hpp

@@ -108,6 +108,8 @@ private:
     void selectStage(int index);
     void onStageSelected();
     Gtk::Widget& buildMainMenu();     // the header menu: new, open, save project
+    void showChangelog();
+    void showAbout();
     Project currentProject() const;
     void applyProject(const Project& project, const std::string& path);
     void clearSources();
@@ -209,6 +211,9 @@ private:
     Gtk::Button* addButton_ = nullptr;
     // Built fresh with the progress window, and null whenever it is closed.
     std::unique_ptr<Gtk::Window> loadWindow_;
+    // Opened from the menu on first use, then shown again rather than rebuilt.
+    std::unique_ptr<Gtk::Window> changelogWindow_;
+    std::unique_ptr<Gtk::AboutDialog> aboutDialog_;
     Gtk::ProgressBar* loadBar_ = nullptr;
     Gtk::Label* loadFileLabel_ = nullptr;
     Gtk::Box sourcesSection_;  // hidden while the chain needs no sources