Install dark icons via execute_process to use cp -a

Rodney Dawes requested to merge dobey/breeze-icons:fix-445489 into master

In order to ensure the dark icons are copied over the light icons which are installed into the dark theme for BUG:444095, we need to execute the cp command ourselves rather than using CMake's internal copy mechanism used by the normal install command, so we use CODE mode of install to execute_process and print an appropriate status message.

BUG: 445489

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