Pass delimiter information to ValueSource

[kwin]

ValueSource may delegate to another ValueSource by returning an expression itself in its getValue(...) method. However that requires that the returned value is enclosed in the delimiters.
However, the delimiters are configurable, so somehow the ValueSource implementation needs to know which ones are detected by the calling Interpolator.

[kwin]

The easiest fix is to add another method Object getValue(String expression, String delimiterStart, String delimiterEnd) which calls by default just getValue(expression). Each interpolator needs to call each ValueSource with valid delimiter arguments.