Comments for expressions

Adds support for comments in QgsExpressions. The syntax is based on the SQL standard

Block comments
----------------------

```sql
    /*
     * This is
     * a multi-line block comment
     */

```

Line comments
-------------------

```sql
    CASE
      WHEN name = 'QGIS' THEN 'Yay' -- Using the good stuff
      ELSE 'Nay' -- You can get it on http://qgis.org
    END
```