Fragment name content assist

[ultraq]

Just like how the plugin can provide expression object suggestions or enumerated value suggestions inside certain attribute processors, provide fragment name suggestions when inside th:include or th:replace processors.

Those aren't the only fragment inclusion processors, so maybe see if I can mark certain processors as fragment inclusion ones, then have content assist inside such processors offer those fragment name suggestions.

Relies first on https://github.com/thymeleaf/thymeleaf-extras-eclipse-plugin/issues/29