use integer sizes for qt

[tspiteri]

Pass integers to Qt functions requiring integers.

[tspiteri]

Without this, pyfdax crashes on Fedora 35 for me.

For context (if it is helpful), my Fedora Qt python packages are:

$ pip list | grep -i qt
PyQt5                5.15.6
PyQt5-sip            12.9.0
PyQtWebEngine        5.15.2

User packages including pyfda:

$ pip list --user
Package    Version
---------- -------
colorama   0.4.4
docutils   0.18.1
migen      0.9.2
mplcursors 0.5.1
numexpr    2.8.1
pyfda      0.6.0b1

[chipmuenk]

I'm using an older version of PyQt, that might explain why I haven't experienced that problem yet under Windows or Linux. But the conversion to int definitely makes sense. Thanks for contributing!