Ensure table name length limit

[effjot]

Except if configured differently at compile time, PostgreSQL allows a maximum length for names of tables, indexes, sequences etc. of 63 characters. This PR limits the table name length to this maximum minus the characters needed for the GiST index suffix.

• The number of characters that can be entered in table name widget is limited by this value.
• If a layer with a longer name is selected, the name is truncated and the user informed about this.

Also, any question marks in the laundered table name (added by the conversion to ASCII for umlauts etc.) are replaced by underscore, which seems safer.

[hdus]

Your PR is merged, thank's a lot for your contribution.