Allow cursors to be linked for plots

[Zoxc]

This adds set_link_cursor to LinkedAxisGroup which allow the cursor to show up in linked plots.

This only handles the default PlotItem case. I think changing PlotItem::on_hover to produce a list of cursors would make sense.

The PartialEq implementation for LinkedAxisGroup also seems a bit questionable given it carries identity.

[blitzarx1]

That would be a nice contribution. Waiting for the feature! Thanks in advance ;)

[emilk]

It would be nice of an example of this so we can test it!

[enomado]

Wanted this feature

[Zoxc]

I did some refactoring and added the ability to link cursors to the linked axis demo.