closes #166

Also moved animation and render methods from Subplot to PlotBase, because we need DockedPlotArea to use animation functions which is required for LinearRegionSelector plot hooks.

remaining todo:

• integrate with image widget, removing existing ipywidget sliders
• bidirectional events with associated ImageGraphic
• garbage collection, make sure events are cleaned up if switching ImageGraphic too
• text to display min and max next to the lines? Will do in another PR since text was refactored
• allow adjusting flank size
• prevent LinearRegionSelector from re-enabling the controller when used in docks

For now this is not easily exposed by any imports and only used by ImageWidget. We can expose after it stabilizes more and if we're happy with keeping this in fastplotlib/widgets.