In fulfillment of #226, this PR adds a BadgedPixmap class that overlays a red/circular "badge" onto an arbitrary pixmap that may contain text, at a relative size, and in a corner of the developer's choice. This is intended to be used to highlight the presence of pending or important information within the application, for example, like so (on macOS):

Note that this is mostly intended for future purposes; for now, a notification "badge" will only be displayed to signal a pending/unread "newscap" message (however, the same functionality may be used for other/additional purposes later on -- e.g., to communicate software updates, to highlight a magic-folder conflict, to remind users to export a Recovery Key, to notify of an expiring subscription to a commercial service provider, and so on).