Refine SpringApplication source types

Update `SpringApplication` so that the `run` methods and constructors
now require `Class<?>` arguments, rather than `Objects`. String based
sources can still be loaded, but must now be set on the `getSources()`
collections. `Package` and `Resource` types are no longer directly
supported.

This change should help IDEs offer better content assist, and will
help integrations with alternative languages such as Ceylon.

Users currently passing in Class references or using the
`spring.main.sources` property should not be affected by this change. If
an XML resource is being used, some refactoring may be required (see the
changes to `SampleSpringXmlApplication` in this commit).

Fixes gh-9170

spring-boot-docs/src/main/asciidoc/using-spring-boot.adoc

@@ -398,10 +398,10 @@ The `Application.java` file would declare the `main` method, along with the basi
 
 [[using-boot-configuration-classes]]
 == Configuration classes
-Spring Boot favors Java-based configuration. Although it is possible to call
-`SpringApplication.run()` with an XML source, we generally recommend that your primary
-source is a `@Configuration` class. Usually the class that defines the `main` method
-is also a good candidate as the primary `@Configuration`.
+Spring Boot favors Java-based configuration. Although it is possible to use
+`SpringApplication` with an XML sources, we generally recommend that your primary
+source is a single `@Configuration` class. Usually the class that defines the `main`
+method is also a good candidate as the primary `@Configuration`.
 
 TIP: Many Spring configuration examples have been published on the Internet that use XML
 configuration. Always try to use the equivalent Java-based configuration if possible.