cmake: Introduce CMake-based build option

Fixes #689.

assets/CMakeLists.txt

@@ -0,0 +1,17 @@
+if(UNIX AND NOT APPLE)
+    find_package(ECM REQUIRED NO_MODULE)
+    set(CMAKE_MODULE_PATH ${ECM_KDE_MODULE_DIR})
+    include(KDEInstallDirs)
+
+    foreach(_i 16 24 32 64 128)
+        install(FILES "freedesktop/appicons/${_i}/zeal.png"
+            DESTINATION "${KDE_INSTALL_ICONDIR}/hicolor/${_i}x${_i}/apps"
+            # TODO: Use `RENAME zeal.png` and get rid of subdirectories.
+        )
+    endforeach()
+
+    # TODO: Generate via zeal.desktop.in.
+    install(FILES "freedesktop/zeal.desktop"
+        DESTINATION ${KDE_INSTALL_APPDIR}
+    )
+endif()