Added enum for filtering placeholders

Linked with dandelion/issues#116
New way to deal with filtering

* The FilteringFeature has been renamed into AbstactFilteringFeature
because there exists a JSP and a Thymeleaf implementation to deal with
HTML generation in a different way
* New IT added for both filtering and filterPlaceholder
* Missing enumeration added in Utils#generateStringRegexFromEnumerations
so that it's now possible to avoid wrapping enum values with quotes for
dt:filterType, dt:filterPlaceholder and dt:sortType
* It is now possible to set filterPlaceholder="none" in case if custom
filtering form : closes dandelion/issues#115
* Closes dandelion/issues#116