UI: factory: access gadget definitions with is_definition() and definition_name()
UI: factory: extract gadget definition inheritance with parent_type_name()
RCORE: guard debug_config_get() against early execution
RCORE: add MarkupParser::seek_to_element()
UI: factory: use XmlNode::parse_xml() pseudoroot to ensure well formed XML
UI: factory: parse <Widget/> element syntax for definitions

* Partially revert "tmpl:define" syntax from commit 6230b78cd2fd008bdafdfedbbe6f7cb8a4bac11c
UI: use the same UI XML syntax for using and defining widgets

* Partially revert "tmpl:define" syntax from commit 6230b78cd2fd008bdafdfedbbe6f7cb8a4bac11c
UI: factory: parse <Argument/> element syntax for definition arguments

* Partially revert "tmpl:argument" syntax from commit 6230b78cd2fd008bdafdfedbbe6f7cb8a4bac11c
UI: use <Argument id="argname"/> syntax in UI XML files

* Partially revert "tmpl:argument" syntax from commit 6230b78cd2fd008bdafdfedbbe6f7cb8a4bac11c
UI: factory: parse property element syntax for widget properties

* Partially revert "tmpl:property" syntax from commit 6230b78cd2fd008bdafdfedbbe6f7cb8a4bac11c
UI: use <Widget.property/> syntax in UI XML files

* Partially revert "tmpl:property" syntax from commit 6230b78cd2fd008bdafdfedbbe6f7cb8a4bac11c
UI: remove unneeded xmlns declarations from XML files
UI: factory: remove lagacy checks
Merge branch 'xml-syntax'

* xml-syntax:
  UI: factory: remove lagacy checks
  UI: remove unneeded xmlns declarations from XML files
  UI: use <Widget.property/> syntax in UI XML files
  UI: factory: parse property element syntax for widget properties
  UI: use <Argument id="argname"/> syntax in UI XML files
  UI: factory: parse <Argument/> element syntax for definition arguments
  UI: use the same UI XML syntax for using and defining widgets
  UI: factory: parse <Widget/> element syntax for definitions
  UI: factory: use XmlNode::parse_xml() pseudoroot to ensure well formed XML
  RCORE: add MarkupParser::seek_to_element()
  RCORE: guard debug_config_get() against early execution
  UI: factory: extract gadget definition inheritance with parent_type_name()
  UI: factory: access gadget definitions with is_definition() and definition_name()