CMake: Define gammaray_pch_core_gui as an INTERFACE library #251

Job	Run time
build (ubuntu-latest, ci-dev-client-only-qt5, 5.15)	4m 10s
build (ubuntu-latest, ci-dev-client-only-qt6, 6.5.*, qtshadertools qtscxml)	5m 22s
build (ubuntu-latest, ci-dev-client-and-ui-qt5, 5.15)	6m 44s
build (ubuntu-latest, ci-dev-client-and-ui-qt6, 6.5.*)	9m 7s
build (windows-latest, ci-dev-client-only-qt5, 5.15)	5m 11s
build (windows-latest, ci-dev-client-only-qt6, 6.5.*, qtshadertools qtscxml)	7m 15s
build (windows-latest, ci-dev-client-and-ui-qt5, 5.15)	7m 35s
build (windows-latest, ci-dev-client-and-ui-qt6, 6.5.*)	10m 46s
build (macos-latest, ci-dev-client-only-qt5, 5.15)	5m 2s
build (macos-latest, ci-dev-client-only-qt6, 6.5.*, qtshadertools qtscxml)	9m 7s
build (macos-latest, ci-dev-client-and-ui-qt5, 5.15)	17m 44s
build (macos-latest, ci-dev-client-and-ui-qt6, 6.5.*)	15m 56s
	1h 43m 59s

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