SqliteTableModel: Remove comments from SQL queries

Remove any single line comments from the SQL queries to fix all those
problems ocurring when a query ending with such a comment is inserted
into the COUNT query.

Executing the query
SELECT * FROM table -- comment
would otherwise lead to this one being executed as well
SELECT COUNT(*) FROM (SELECT * FROM table -- comment);
This is obviously invalid SQL and therefore returns no data, sometimes
even crashing the application, even though the original statement by the
user is perfectly fine.

The code used in this commit is a bit of a workaround and should be
replaced as soon as there is a more complete SQL parser supporting SQL
comments in our grammar tools.

Closes #31.