Give Effector its own icon

An E split into its colour channels, with two slices slipped sideways,
replaces the generic applications-graphics icon in the desktop entry and
About Effector, with a one-colour symbolic version for panels. Both are
installed into the hicolor theme; a build run straight from the build tree
falls back to the copy in the sources. The version moves to 0.5.3.

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

CHANGELOG.md

@@ -4,6 +4,13 @@ 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.
 
+## [0.5.3] - 2026-09-26
+
+### Added
+- Effector has its own icon, an E split into its colour channels, in the app
+  menu, the dock and About Effector; a one-colour version goes with it for
+  panels.
+
 ## [0.5.2] - 2026-09-26
 
 ### Changed