cubicweb #16164880 Enhance selectable rtags [done]

Since some releases, rtags (structure underlying uicfg) have selector and may be copied using something like:

new_rtags = deepcopy(original_rtags)
new_rtags.__module__ = __name__
new_rtags.__select__ = custom_selector

The problem is that starting from that, both rtags wil diverge and changes in original_rtags won't be considered, while we usually want to set a few specific rules only in new_rtags. To fix this problem, I propose to introduce the notion of "derivated/parent" rtag, eg:

new_rtags = original_rtags.derive(__name__, custom_selector)

Beside easier copying, when using the above method changes in original_rtags which are not overriden by new_rtags will be considered since it only hold its specific rules but look among its parent chain for non-found keys.

done in<not specified>
load left0.000
closed by#0849a5eb57b8 [rtags] Allow to 'derive' rtags
patch[rtags] Allow to 'derive' rtags [applied]