maint: Comment fixes

* dietncl/filter/region.lua: Likewise.
* dietncl/filter/transition.lua: Likewise.
maint: Fix bugs in xmlsugar's clone() and children()

* dietncl/xmlsugar.local (children): Make it safe to remove elements
while iterator is being traversed.
(clone): Fix parent linkage.
* tests/test-xmlsugar-children.lua: Check if it is safe to remove
elements during iteration.
* tests/test-xmlsugar-clone.lua: Check parent linkage.
maint: Fix bug in 'import' filter

* dietncl/filter/import.lua (import_base): Remove elements from the
cloned base before inserting them into the host base.
maint: Add get_last_gen_id() to nclaux

* dietncl/nclaux.lua (get_last_gen_id): New function; returns the last
XML-ID generated for a given document.
* tests/test-nclaux-gen-id.lua: Checks the new function.