Revise PropertyBuilder methods for default values

[ljacqu]

The defaultEntry method on MapPropertyBuilder should be renamed to make a clearer distinction between it and PropertyBuilder#defaultValue. The former amends values to a default while the former overwrites any previous values.

• PropertyBuilder#defaultValue sets the default value (overwrites)
• MapPropertyBuilder#defaultEntry puts to the existing map it keeps

Additionally, it would be nice to have a method like addToDefaultValue for builders that create a property of Collection type (SetPropertyBuilder, ListPropertyBuilder)

[ljacqu]

New structure for property builders:

• The method defaultValue sets the entire default value for a property; it may only be called once, and not in combination with other methods that modify the default value.
• Property builders for collections, maps and arrays have another method addToDefaultValue that allows to define one entry of the default value. This method can be called repeatedly.