This change improves the support for auto-registration of FreeMarker,
Velocity, and Tiles configuration.

The configuration is now conditional not only based on the classpath
but also based on whether a FreeMarkerConfigurer for example is already
present in the configuration.

This change also introduces FreeMarker~, Velocity~, and
TilesWebMvcConfigurer interfaces for customizing each view technology.

The WebMvcConfigurer can still be used to configure all view resolvers
centrally (including FreeMarker, Velocity, and Tiles) without some
default conifguration, i.e. without the need to use the new
~WebMvcConfigurer interfaces until customizations are required.

Issue: SPR-7093

This commit improves and completes the initial MVC namespace
view resolution implementation. ContentNegotiatingViewResolver
registration is now also supported.

Java Config view resolution support has been added.
FreeMarker, Velocity and Tiles view configurers are registered
depending on the classpath thanks to an ImportSelector.

For both, a default configuration is provided and documented.

Issue: SPR-7093