XML/markup syntax should highlight well known templating syntax

[rbanffy]

XML is often used in templating contexts, with values being replaced by other information that's available externally. This need became apparent while working with JMeter, which uses ${..} as template variable markers, but it could be used in other contexts as well.

One possible solution would be to have MARKUP_TEMPLATE_ token types added to TokenTypes, along with the corresponding change in behavior and the respective highlighting added to the theme resources.

The corresponding ticket in JMeter's tracker is https://bz.apache.org/bugzilla/show_bug.cgi?id=64401