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/core/version.hpp.in

@@ -0,0 +1,16 @@
+#pragma once
+
+// Generated by CMake from src/core/version.hpp.in; edit that, not the copy in
+// the build tree. The version comes from the project() line of CMakeLists.txt
+// and the changelog from CHANGELOG.md, both read at configure time.
+
+#include <string_view>
+
+namespace effector {
+
+inline constexpr std::string_view kVersion = "@PROJECT_VERSION@";
+
+// CHANGELOG.md as it stood when the build was configured, as Markdown.
+inline constexpr std::string_view kChangelog = R"changelog(@EFFECTOR_CHANGELOG@)changelog";
+
+}  // namespace effector