Package Effector for Arch and give it a desktop entry

packaging/arch/PKGBUILD builds the tagged release from this repository
and installs it with pacman; the version test keeps its pkgver in step
with CMakeLists.txt. A desktop entry puts the interface in the app menu.

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 0.5.0 LANGUAGES CXX)
+project(effector VERSION 0.5.1 LANGUAGES CXX)
 
 set(CMAKE_CXX_STANDARD 20)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
@@ -77,6 +77,8 @@ target_link_libraries(effector PRIVATE effector_gui)
 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)
 
 # --- tests --------------------------------------------------------------------
 include(CTest)