Clarify ignore missing to support ignore missing fields in source bean and/or destination bean

[slemesle]

User should be able to ignore fields from source bean or from destination bean separately.

Introduce an IgnoreMissing enum:

public enum IgnoreMissing {
    /**
     * Ignore fields appearing in destination and missing in source bean.
     */
    SOURCE,

    /**
     * Ignore fields appearing in source and missing in destination bean.
     */
    DESTINATION,

    /**
     * Ignore all missing fields from source and destination bean
     */
    ALL,

    /**
     * Report all missing fields from source and destination bean
     */
    NONE,

    /**
     * If setted on {@link @Maps} then {@link @Mapper} setting is applied otherwise {@code NONE} is applied.
     */
    DEFAULT;
}

This enum can be used in @Mapper and @Maps using the withIgnoreMissing parameter.

With this new behavior Selma will allow to map a DTO containing a subset of the fields appearing in the source bean and reverse.

This deprecates the old ignoreMissingProperties parameter.