Fix SQL Console problems

This (almost) fixes all the issues listed in #643:
* Functions are listed only once in the SQL console's function list
* The comment problem (`--` at the end of the line rather than at the beginning) is fixed

We now generate function signatures using
`DatabaseMetaData#getProcedureColumns`, and sort them by increasing length.
This works with both H2 and PostGIS. The solution is good, but not perfect.
Problems:
* Doesn't work for `ST_Accum` (aggregate function). @nicolas-f An H2 patch is needed.
* When we use `Geometry...` in Java, the generated signature displays `ARRAY`.
* It's practically impossible to handle return types.
* PostGIS parameter names are not always recognized. When they're not, they're listed as `$1`, `$2`, etc.

See also @nicolas-f's patch here:
    https://groups.google.com/forum/#!topic/h2-database/6HlB7fXLggQ