configitems: add a devel warning for extensions items overiding core one We do not want such case to pass silently. In the future we'll likely have useful tool for an extension to alter the existing definition in core.

configitems: add an official API for extensions to register config item Extensions can have a 'configtable' mapping and use 'registrar.configitem(table)' to retrieve the registration function. This behave in the same way as the other way for extensions to register new items (commands, colors, etc).

configitems: extract the logic to build a registrar on any configtable Having the logic available independently from the mapping used is a necessary step toward extensions support.