Optionally install icons

This is required for Flathub.
Also rename icons to add group (<size>-<group>-<name>.<ext>).

CMakeLists.txt

@@ -113,6 +113,13 @@ kconfig_add_kcfg_files(kcalc_KDEINIT_SRCS kcalc_settings.kcfgc )
 file(GLOB ICONS_SRCS "${CMAKE_CURRENT_SOURCE_DIR}/icons/*.png")
 ecm_add_app_icon(kcalc_KDEINIT_SRCS ICONS ${ICONS_SRCS})
 
+# Conditionally install icons for Linux as they may not be provided by the user theme
+option(INSTALL_ICONS "Install icons" OFF)
+if (INSTALL_ICONS)
+    include(ECMInstallIcons)
+    ecm_install_icons( ICONS ${ICONS_SRCS} DESTINATION ${KDE_INSTALL_ICONDIR} )
+endif()
+
 kf5_add_kdeinit_executable( kcalc ${kcalc_KDEINIT_SRCS})
 
 target_link_libraries(kdeinit_kcalc