Since the vast majority of message boxes should be modal, require
the parent window to be passed in, which can be nullptr for the
rare case that they don't. By it being the first argument, the
default arguments also don't need to be explicitly stated in most
cases, and it encourages passing in a parent window handle.
Fix up several message boxes that should have been modal.
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. :^)
