Use getExpressionString/getFilterString/getTextString in PHP MapScript

[dmorissette]

In bug 1428 it was pointed out that SWIG MapScript has:

- class.getExpressionString()
- layer.getFilterString()
- label.getTextString()

and PHP MapScript has:

- class.getExpression()
- layer.getFilter()
- missing label.getTextString()

We should synchronize both interfaces in 5.0.

Would it make more sense to add the String suffix in PHP, or to remove it in the
SWIG wrappers? Note that in both SWIG and PHP, the set*() methods don't have the
string suffix (i.e. setFilter(), setExpression(), etc.)