Filterable backcompat: Restrict application of filterPlaceholder to generated filter inputs. Fixes #6355 - Filterable: Placeholder on search input is overridden incorrectly
Integration tests: Filterable: Removing placeholder-related markup, because we do not test the placeholder handling - it is deprecated.
Integration tests: Filterable: Moving filter_core.js to filterable_core.js to stay in line with naming conventions.