Database Purge; move "Master Clear" option to a button at the bottom of the form

The Database Purge "Master Clear" option has been moved from a checkbox to a button at the bottom of the form.

At the same time the Business Layer has been updated to use a separate "ExecuteMasterClear" function in the finDatabasePurge object rather than the existing "Execute" function.

This means it is a breaking change as the finDatabasePurge.MasterClear property has been removed.