Change DTM presence detection in dependant plugins

Earlier, several plugins used to check for the presence of the DTM
plugin at initialization time.

That test was not reliable, because plugin loading order is arbitrary,
(so maybe the plugin is going to be loaded later), furthermore, even
if the plugin is present, we can not be sure that the DTM document
access policy is actually used.

So now I have removed those tests at initialization time, and added
runtime tests, which look for availability actual methods instead.
DomTextMapper plugin: support the `skip` option