system.defaults.screencapture.location should create the directory

[nalzok]

When we set system.defaults.screencapture.location to a non-existent directory, it will silently fail and the screenshots will end up in the default location $HOME/Desktop/. It would be nice if we could execute an additional mkdir -p to make sure it exists. In addition, we need to consider whether it makes sense to "migrate" the screenshots when the user updates system.defaults.screencapture.location to a new location.

There appears to be a related but abandoned pull request: #566