Give the completion-list items to the kate completion-model hiearchically, grouping items that have common role-values like InheritanceDepth and ArgumentHintDepth together.

Kate does not need to query the values that are inherited from the parent nodes any more, and in future it may even use the pre-computed grouping.

This makes the completion-list show up instantly, instead of waiting for seconds with blocked UI, even if it is very long.

This needs up-to-date kdelibs.