Add a clear button (default for now) to clear the SQL query field.

Move query button to the left, to avoid conflict.

Fixes #5
Added AsyncResult class to provide type-safe way to return a Throwable or a typed result from an Async task.

Catch throwables while executing SQL queries, and report them with a Toast in the main thread.

Add a query to the history only if it succeeded.

Don't try to import a null history.

Fixes #8
Fixes #9
Fixes #10