Support for "Docbase" document fields in API

The Document Type Editor provides a "Docbase" field type which can be used in document types to store the UUID of another content item (sort of a weak reference).

Currently the API does not provide an easy way to resolve a link stored as Docbase field.

Ard proposed:

Perhaps the HST can provide some helper utility for it so developers
can easily account for it in their end project beans. Where we now
have

public <T extends HippoBean> T getLinkedBean(String relPath, Class<T>
beanMappingClass) {

we could add something like

public <T extends HippoBean> T getLinkedBean(UUID uuid, Class<T>
beanMappingClass) {

Developers should then implement the getter for a
linked bean through docbase themselves, for example

public HippoBean getLinkedBean() {
String docbase = getProperty("gettingstarted:docbase", null);
try

{ UUID uuid = UUID.fromString(docbase); return getLinkedBean(uuid, HippoBean.class); }

catch (IllegalArgumentException e) {
log.info("Cannot get linked bean for docbase that is not a
uuid {}", uuid);
return null;
}
}