This commit is aimed at cleaning up the filtering code by changing the
filtering idiom/pattern used. While the old code used a "check then
do" approach, the new code does a "grab then assimilate".
The main benefits are that:
* the code duplication that used to exist has now been removed, making
it easier to add new channel types for data
* a recursive "peeking" ability now means that the old problems with
data existing deep in the tree (i.e. figuring out whether a channel
should be shown based on whether it will have any descendents) should
now work much better than before.
In the process, I've found and fixed a few previously unnoticed bugs
with how some channels were constructed, so hopefully things work a
bit better now.
TODO's:
* Action-Group filtering stuff hasn't been refactored yet. This was
causing some grief in the past, so I still need to check this
carefully.
* Material Nodes support (missing in trunk) should be easy to slot in
now :)
10d775df3d