Blender Git Loki

File Browser: Various fixes and enhancements to 'autoscroll to item' feature.

Initial trigger to this work was T65782, requesting faster autoscroll
when current folder contains thousands of items. That was a fairly
simple change, just needed to make scrolling steps variable based on
'distance' between current position and desired one.

But several other issues showed up while working on this, among the most
annoying ones, the scrolltimer could keep running forever in some cases,
failing to detect properly an 'end condition', we could even get some
'bouncing' in extreme corner cases, edited item was not always properly
visible in the end, etc.

So as usual with UI, this ended up in a frustrating equilibrium game of
finding the optimal solution among several tradeof, taking unexpected
large amount of time... At least new code seems to work OK in
all possible (reasonable) cases, that will do for now.