Make SchemaName on DefaultMigration overridable

[remyblok]

We are using migration to add stuff to the database. We inherit our migrations from DefaultMigrator. This class has a property SchemaName. This property always contains a value stating with "bcms_". We want our tables in a schema prefix with our own product name, so that we can easily distinguish what is bcms and what is ours.
The SchemaName-property cannot be overridden so we are stuck with the "bcms_" prefix and so we do not use the SchemaName-property at the moment. It seems that BetterCMS also does not use the property for anything other then it Migrations. But in the future this might change for some reason. if then the SchemaName is incorrect it would lead to issues in our software.

Therefore I propose to make SchemaName in the DefaultMigrator-class virtual, so that we can override it and set the total schema name to something we want. Thanks!