automatically scroll the layer UI vertically in order to maintain the current layer visible, so that selecting layers with the Up and Down arrows doesn't end up with the active one being out of view if you have many layers.

...and listen to it in timeline.cpp instead of the old signal.

this fixes an issue with the current layer scrolling implementation where clicking a barely-visible layer would move it due to the scrolling happening while the pointer was still being pressed. now clicking layers no longer triggers the scrolling, only the Up and Down arrow keys do.

note that I didn't remove the original signal emission from the goto methods not to break anything else.

This reverts commit 662ba4f.

This prevents the timeline cells from swapping layers while the scrollbar emits changes.

So we can reset the 'mScrollingVertically' properly.