Add "Get CSS Property Value" keyword to retrieve computed CSS values #36

Job	Run time
test_config (latest, 3.13.10, 7.4.1, 4.39.0, chrome)	1m 6s
test_config (previous, 3.12.12, 7.3.2, 4.38.0, chrome)	1m 1s
test_config (older_rf_version, 3.11.14, 6.1.1, 4.37.0, chrome)	1m 8s
test_config (older_rf_version, 3.11.14, 6.1.1, 4.37.0, chrome)	59s
test_config (latest, 3.13.10, 7.4.1, 4.39.0, chrome)	1m 5s
test_config (previous, 3.12.12, 7.3.2, 4.38.0, chrome)	1m 0s
	6m 19s

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