grc: Fix crash when using function probe with BokehGUI

[Notou]

The function probe block creates a new worker thread to do it's thing, this means that the callback to update the value of the block is executed in that separate thread.
The bokeh framework doesn't allow that, so using a probe block value in a BokehGUI block causes a crash.
This PR solves it by giving the callback function to the bokeh framework so it can execute it in its own thread.
I did not know how to check whether QT or bokeh is active from the yaml file, so I made a nested try.

This can be backported to maint-3.8.

[michaelld]

small tweak, but otherwise LGTM

[michaelld]

I tagged a few other GR devs to see what they think ...

[michaelld]

LGTM

[michaelld]

@mormj you around to check out this PR? Should be quick ...

[mormj]

I've actually never tried out the bokeh framework, so this one I'll have to give a try later

[michaelld]

@mormj ping ... any progress? I've ever tried the boken framework either ... but the code looks good & if the author says it does the trick I'm willing to go with that.

[mormj]

Trying to get bokeh set up, but ran into this (among others):
gnuradio/gr-bokehgui#19

Also, gr-bokeh does not currently compile with master branch gr. Not sure the depth of changes needed

[Notou]

I have not yet pulled the 3.8 port to the gnuradio gr-bokehgui repo.
I the mean time, you can use this fork: https://github.com/Inria-Maracas/gr-bokehgui

[Notou]

And now it's done. You should be able to build with maint-3.8. Master is not yet supported since the change in shared pointers.

[Notou]

@mormj ... Any news on this part? It should even work on master (at least related to the shared pointer changes).

[mormj]

I still haven't gotten gr-bokeh to work yet, but I pulled in this change and verified it didn't break the function probe, so I go ahead and merge.