Switch implementation from Field to Element

[martinfrancois]

As of FormsFX 1.3.0, introduced by dlsc-software-consulting-gmbh/FormsFX#28 a Field is now a subclass of Element. From PreferencesFX 2.1.0, the included dependency of FormsFX has been updated to 1.3.0, but still only Field is supported and not Element.
By using Field we are limited in terms of the features of FormsFX we and also the API user can use and thereby benefit from.

This change would involve replacing all occurences of Field with Element and to also expose Element instead of Field externally. Currently, a quick fix is in place (#39), which casts the Element objects to Fields. Going with Element internally instead of Field eliminates the need for a cast and opens up additional features which can then be used for custom controls and also to expose additional functionality with PreferencesFX.

Migrating to the new architecture of FormsFX makes sense, but since PreferencesFX makes heavy use of FormsFX, also using custom renderers and controls to change a lot of the behavior of FormsFX, this is not an easy task, while still preserving the original functionality as currently implemented by PreferencesFX.