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>

CHANGELOG.md

@@ -0,0 +1,73 @@
+# Changelog
+
+Every release of Effector, newest first. The version lives in one place, the
+`project()` line of `CMakeLists.txt`, and each release is a `vX.Y.Z` tag on the
+commit that made it. The interface shows this file under Changelog in its menu.
+
+## [0.5.0] - 2026-09-26
+
+### Added
+- A version number: `effector --version` prints it, and About Effector in the
+  menu shows it.
+- This changelog, readable from the interface under Changelog in the menu.
+- Open Recent in the project menu, listing the projects opened or saved here.
+- An optional shadow or glow under the image overlay, with an offset, size,
+  spread, colour and opacity.
+- Reordering the chain by dragging a row.
+
+### Fixed
+- The params' scrollbar no longer covers the spin buttons.
+
+## [0.4.0] - 2026-09-23
+
+### Added
+- Eight chaotic effects: slit shift, channel split, pixel sort, corruption,
+  shatter, warp, bit crush and feedback. None needs sources, so any of them can
+  lead a chain.
+- The sidebar split into Chain and Setup tabs.
+- A rotation for the text effect's placement.
+- Text stages labelled in the chain list with their words.
+
+### Changed
+- The preview stays centred however long the status line gets.
+- The status line stays short, with the full chain and setup in its tooltip.
+- The chain/params split fits the chain's rows until it is dragged; a
+  double-click on the handle fits it again.
+- The mouse wheel scrolls past spin buttons that do not have the focus.
+
+## [0.3.0] - 2026-09-19
+
+### Added
+- Named styles: several text stages can share one font and colour, and editing
+  the style moves every stage that follows it.
+- Switching a chain stage off without taking it apart, in the interface and
+  with `--disable NAME` on the command line.
+
+### Changed
+- The interface opens on an empty chain, and shows the sources only when the
+  chain uses them.
+
+## [0.2.0] - 2026-09-18
+
+### Added
+- Effect chains: stages cascade, each handed the previous one's output, built in
+  the sidebar or with repeated `-e`.
+- Blur, film grain, black and white, image overlay, text and tiles effects.
+- Projects saved and opened as plain JSON.
+- Standard web and print canvas formats (`--format`, `--dpi`).
+- Effects declare whether they need source images, so a chain led by blur
+  renders with none loaded.
+
+### Changed
+- The chain comes first in the sidebar.
+
+### Fixed
+- The canvas size is checked before it is narrowed to an int.
+- Projects are written and read in the C locale, and keep the seed whole.
+
+## [0.1.0] - 2026-09-18
+
+### Added
+- The C++20 port of the Python prototype: a GTK 4 interface and a headless
+  command line over pluggable effects, starting with scanlines.
+- Source images load in the background, with a progress window and Cancel.