"docked subplots" now work on any side of a subplot, this will allow making a histogram LUT widget similar to pyqtgraph's

things to consider:

1. make a "view area" class which is kinda like a Subplot, but they can only exist as children of a subplot and are shown within the original Subplot viewport area. Also this way Subplot itself can mostly just keep plotting relevant capabilities.

PlotArea:
    attrs:
        parent
        position
        renderer
        camera
        controller
    methods:
        get_rect()
        set_viewport_rect()
        render()
        add_graphic()
        center_graphic()
...

1. Start a module for widgets like the HistogramLUTWidget which are more complex than just single Graphic items.