EditTableDialog: Don't quote function in default value when marked '=()'

When entering a default value like "=(strftime('%s','now'))", don't
quote it but remove the '=' character and use the following expression
as default value.

See issue #166.
cipher: Fix leak and database being locked when opening a normal DB file

Fix a memory leak and make sure the database is being fully unlocked
when opening a normal, not-enrypted database file.