During app teardown, the Application object may be destroyed before
something else, and so having Application::the() return a reference was
obscuring the truth about its lifetime.
This patch makes the API more honest by returning a pointer. While
this makes call sites look a bit more sketchy, do note that the global
Application pointer only becomes null during app teardown.
Instead of using a TableView to show the layer stack, let's build a new
custom widget for this purpose and give it some neat features!
This patch also introduces an ImageClient interface for Image to notify
interested parties about things happening. The LayerListWidget is the
first ImageClient and listens for "layer added" and "layer removed"
notifications. :^)
