Use the new TemplateManager to determine if a template exists as custom displayer

Currently, to determine if a custom displayer exists for a property, various places are checked (XWiki document in the current wiki, XWiki document in the main wiki and template file).

However, when determining if a template file exists to be used as a custom displayer, XWiki.evaluateTemplate(...) is used and the result is verified if it is empty.

With the addition of the TemplateManager (XWIKI-11252), we can now check if a template exists (TemplateManager#getTemplate(...)) without having to render it, so we can avoid a costly and possibly dangerous operation.