Refactor QT

[giulioungaretti]

The current QtPlot is painful to work with:

• currently impossible to save the plots
• or to handle qt related in any idiomatic way
• and much more

First step will be merging #290 ! Then polishing all the things.

[giulioungaretti]

Note to self: this may require major changes:

• base class is too limiting
• matplotlib and qt have totally different concurrency models
• in general using the js timer to control QT is a bit suicidal
• qt (actually everything data) shines when it's a dumb listener, not an active puller

[peendebak]

@giulioungaretti @MerlinSmiles The new pyqtgraph (0.10) has been released, so merging #290 is possible.

One issue is that qt4 is still hard-coded, e.g.

from PyQt4 import QtCore, QtGui 

I suggest replacing this with qtpy:

from qtpy import QtCore, QtGui

(als some small changes in other places)

@MerlinSmiles Can you make a new branch outside the main qcodes repo (which is read-only now). I can then make a PR for the qtpy changes.

[giulioungaretti]

@eendebakpt yep! and yep

I will just fix the obsolete things in #290 !

[MerlinSmiles]

@peendebak i just had a discussion with @giulioungaretti , it seems as merging this does not work without multiproccessing (default settings now), or with severe limitations in updating the GUI :( maybe @giulioungaretti finds a fix though 😮

[giulioungaretti]

Note to me and @jenshnielsen this is more and more releavant, as we need to tackle qdev's request.
Although maybe easier to start from scratch and not use that horrible plot_object thing.