Fix order of property sources

The current order of system property sources (cf. documentation) is rather unnatural, because:

1. It gives a higher priority to environment variables than Java system properties. Java system properties apply to a single JVM, whereas environment variables might be shared between processes.
2. It mixes up property sources accessible to system administrators and those accessible only to programmers (`log4j2.component.properties`).

IMHO Log4j should prioritize the sources accessible to system administrators over `log4j2.component.properties`, hence allowing them to easily override the defaults established by developers. It should also prioritize Java system properties over environment variables. This is what, e.g. Spring Boot does. I propose the following order:

1. System properties,
2. Environment variables,
3. `log4j2.component.properties` as failover.

While technically this would be a breaking change, the official property source order never worked and bug LOG4J2-3193 was filed only recently. This proves that almost no one configures the same property in more than one source.