Issue #9595: Restore active layer/mask highlight on select

This patch attempts to resolve #9595 (closed) (thanks @Uzugijin for the notification!)

When multi-selection was added to the Layers dialogue, parts of the selection code from 2.10 were commented out because of conflicts with the Shift key being pressed. This seems to have also impacted the active layer highlight in some cases. This patch adds another call to gimp_layer_tree_view_update_borders () (which redraws the active layer border) to compensate and make the layer updates more consistent.

@pixelmixer Hey! Does this help with the issue you reported?