Remove weak string dependency for determining template mode nature

[danielfernandez]

2.0.18 introduced the org.thymeleaf.util.TemplateModeUtil class, which includes methods for determining whether a template mode is used for HTML or for XML rendering.

This helps in, for example, determining how attributes like multiple, selected, etc. should be rendered (multiple="multiple" in HTML, multiple="something" in XML).

But this is too dependent on a weak mechanism like String comparison (checking whether a word like XML is contained in the template name).

A good solution seems to modify the ITemplateModeHandler, adding some kind of method that can expose the nature of the template mode.

[danielfernandez]

Template modes are specified in Thymeleaf 3.0 as an enum. See #391

Already in 3.0.0-SNAPSHOT