app: push a temporary status when picking layer with alt-midclick.
Though the layer list will also show updated, it is much easier to look at the layer name in the status bar whose position never changes. Anyway it makes sense to just show a temporary status info message giving the picked layer name, making it all the easier to find the layer you are looking for. (cherry picked from commit 496bc02b)
Showing with 25 additions and 4 deletions