Add functionality to evaluate string variables with regular expressions in TextMarker

TextMarker rules are only able to test on the exact values of features. Especially for string features, a more flexible way to compare or test string values (e.g., with regular expressions) is desirable. This functionality can easily be added if the REGEXP condition also works on string variables instead of only for the matched text. The value of a string feature can therefore be stored in a variable, which is then evaluated with a condition in another rule.