Resolve the watermark image name like the splash screen image #35

DockablePlotWidget only accepts a file path (or a Qt resource path) in the watermark_image_path option, whereas the splash screen image (splash_image_path) may be given by its name, looked up in the image paths registered with guidata.

An application therefore has to store the absolute path of its own watermark image in the option, which depends on the installation (see DataLab-Platform/DataLab#462).

Proposal: resolve the watermark image name like the splash screen image (get_image_file_path), and load it with get_pixmap so that its @2x variant is used on high-DPI displays.