Add interactive matplotlib selector to PolygonPixelRegion

[dhomeier]

As next part of the work on interactive regions #391 this PR adds an as_mpl_selector widget to the PolygonPixelRegion class to allow moving and resizing through the draw_box functionality added to PolygonSelector in matplotlib/matplotlib#21830, as well as reshaping by direct manipulation of the vertices. The draw_box handles are working analogously to the selector methods for Rectangle and Ellipse PixelRegions.
Rotation is not yet included, but can be added once the mpl PR or a follow-up supports this. A centroid method is already included to provide a robust and rotation-invariant reference point.
The connected PolygonSelector is instantiated in a rather pedestrian manner, using several private methods, which should perhaps be replaced by a public method on the matplotlib side. using the new verts.setter method for updating the vertices; tiny remaining quirk is the bounding box not being displayed after creation, but only after the first event inside.

[dhomeier]

The last commit adds a method to set the rotation angle on a given Polygon instance as polygon.rotation = 30 * u.deg, and update the selector from it, if present (no test for the latter as the mpl PR is not merged) – as kind of a template for how rotation from the selector might eventually be implemented.

[pllim]

If #500 is merged first, the doc needs to be updated here too.