Fixes #36 Remove filter doesn't work on alphanumeric navtable

Uses the getRecordset helper method instead of trying to get the
source from the layer, and remove the filter with the clearSelection
instead of creating a new empty selection

The instaceof check of AlphanumericData has been also removed, and
tested against shp, dbf, postgis-layers and postgresql-tables
Extract method refactoring related to #36