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>

CMakeLists.txt

@@ -1,5 +1,5 @@
 cmake_minimum_required(VERSION 3.20)
-project(effector VERSION 0.5.2 LANGUAGES CXX)
+project(effector VERSION 0.5.3 LANGUAGES CXX)
 
 set(CMAKE_CXX_STANDARD 20)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
@@ -70,6 +70,9 @@ add_library(effector_gui STATIC
 )
 target_link_libraries(effector_gui PUBLIC effector_core PkgConfig::GTKMM)
 target_compile_options(effector_gui PRIVATE -Wall -Wextra -Wpedantic)
+# Where a build that was never installed finds its own icon (see app.cpp).
+target_compile_definitions(effector_gui PRIVATE
+    EFFECTOR_SOURCE_ICON_DIR="${CMAKE_CURRENT_SOURCE_DIR}/data/icons")
 
 # --- application --------------------------------------------------------------
 add_executable(effector src/main.cpp)
@@ -79,6 +82,8 @@ target_compile_options(effector PRIVATE -Wall -Wextra -Wpedantic)
 install(TARGETS effector RUNTIME DESTINATION bin)
 # The launcher entry, so the interface shows up in the desktop's app menu.
 install(FILES data/org.effector.Effector.desktop DESTINATION share/applications)
+# The icon it names, in full colour and as a one-colour symbolic for panels.
+install(DIRECTORY data/icons/hicolor DESTINATION share/icons)
 
 # --- tests --------------------------------------------------------------------
 include(CTest)