Optimizations for very large completion-lists(see KDevelop):
- Allow optionally passing data to kate hierarchically, so that grouping can take place without querying each item for 3 values.

Parent-nodes define data common for all sub-nodes(see interface documentation).
- Change the vertical scroll-mode back to ScrollPerItem. ScrollPerItem is buggy while scrolling down the list, but ScrollPerPixel needed Qt to compute the size of each completion-item, which is extremely slow for very long lists.
- Remove 2 old files.

Now the completion-list in kdevelop shows instantly instead of waiting for seconds, even when it's very long.