Refactor makeValidHtml into PluginUtils for better reusage

The current approach for MPLUGIN-43 and especially MPLUGIN-73 is suboptimal: These changes are limited to the PluginXDocGenerator although other components have similar needs (e.g. PluginReport from the maven-plugin-plugin which needs to print the mojo description, too).

The patch suggests to move makeHtmlValid() and decodeJavadocTags() into PluginUtils for better reusage among PluginXdocGenerator and PluginReport. Alternatively, these functions could be employed by the descriptor extractors instead of the report generators to beautify the strings right at the beginning of their processing, making them out-of-the-box viewable in beautified format for all consumers of the mojo descriptor.