Tidy GUI folder

[cjmayo]

This works through the fract4dgui folder:

• using keyword arguments with Gtk constructors where possible
• replacing deprecated Gtk.Hbox and Gtk.Vbox
• converting instance variables to local variables where possible
• removing pylint: disable=no-member where possible
• enabling the whole folder to pass all flake8 tests (except E402 for files with gi.require_)

[codecov[bot]]

Codecov Report

Merging #223 (a7fd7e5) into master (1679515) will decrease coverage by 0.23%. The diff coverage is 94.83%.

@@            Coverage Diff             @@
##           master     #223      +/-   ##
==========================================
- Coverage   86.22%   85.98%   -0.24%     
==========================================
  Files          57       57              
  Lines       10619    10409     -210     
==========================================
- Hits         9156     8950     -206     
+ Misses       1463     1459       -4     

Impacted Files	Coverage Δ
fract4dgui/AVIGen.py	10.58% <0.00%> (ø)
fract4dgui/main_window.py	63.88% <ø> (ø)
fract4dgui/painter.py	91.30% <ø> (ø)
fract4dgui/preferences.py	76.61% <ø> (ø)
fract4dgui/utils.py	82.81% <ø> (ø)
fract4dgui/gtkfractal.py	82.28% <28.57%> (-0.03%)	:arrow_down:
fract4dgui/application_widgets.py	90.32% <66.66%> (ø)
fract4dgui/application_window.py	89.78% <96.42%> (-0.54%)	:arrow_down:
fract4dgui/settings.py	80.26% <97.77%> (-1.76%)	:arrow_down:
fract4d/fractconfig.py	95.00% <100.00%> (+2.01%)	:arrow_up:
... and 11 more

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