Add RenderCanvas.set_cursor()

[kushalkolar]

followup from pygfx/wgpu-py#595

I was just looking into how to set or hide the cursor on all the GUI backends, I can do this after the big pygfx/wgpu-py#618 🚀

my idea is just to have a settable property: canvas.cursor = "name", we make a lookup table that maps "name" to the corresponding cursor for that GUI backend.

[hmaarrfk]

Other aspects that m8ght be interesting is setting the cursor position in xy