BUG: Fix ref cycle problem

[larsoner]

This code:

import gc
import time
import pyvista as pv
from pyvistaqt import BackgroundPlotter
from pyvista import examples
vol = examples.download_knee_full()

for _ in range(10):
    plotter = BackgroundPlotter()
    plotter.add_volume(vol)
    plotter.show()
    plotter.close()
    plotter.deep_clean()
gc.collect()
time.sleep(2)

Run with:

mprof run mem.py; mprof plot

Produces this on master:

And this on this PR:

Getting rid of volume was enough to fix it, but I figure getting rid of textactor is also a good idea.

[codecov]

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[akaszynski]

Nice fix. Thanks for the memory plot for proof!