Add menu item to allow attaching other databases

Add a new menu option which allows attaching other databases. Doing so
by running the SQL code by yourself using the Execute SQL tab won't work
because the SQL tab creates a restorepoint and attaching databases while
being in a transaction is not allowed.

Note that this commit misses quite a few features: In case of name
conflicts the UI may break, there is no way to tell which databases have
been attached and the attached databases are not stored in the project
files.

See issue #100.
Silence some warning

Closes #102.