PlotWidget: Added sigBackendChanged

[vallsv]

Added sigBackendChanged to PlotWidget

Related to #3876

Closes #3883

Changelog:

• Added sigBackendChanged to PlotWidget

[vallsv]

I was thinking of added current, previous params to the signal.

Usually if you don't do that, the user of the API have to store the backend reference on its side. So i think it could be done without much problem.

[t20100]

LGTM

The OpenGLAction can be improved with this

[vallsv]

That's a good idea i have updated the stuff.

[t20100]

LGTM!
I removed OpenGLAction.__state since it is no longer used.

[t20100]

I was thinking of added current, previous params to the signal.

If it's needed, why not.
It's not Qt-like but some other signals of PlotWidget already provides current, previous params.

But this means you'll give back a previous backend that is no longer used by a PlotWidget, not sure what it's useful for, and risky if used.

[vallsv]

Yes, that is why i am kind of reluctant. In this case we could use 2 signals like aboutToConnect/aboutToDisconnect, but i feel like it's overkill.

[t20100]

aboutToConnect/aboutToDisconnect, but i feel like it's overkill.

I agree.

sigBackendChanged can also only provide the current backend, that more Qt-like and does cause trouble with giving access to a disconnected backend.

It's fine as it for me.