Add option to disable exit dialog

[samuel-w]

Fixes #680.

Tested by setting is_system_tray_available to False

[codecov-io]

Codecov Report

Merging #681 (70222e8) into master (19b4946) will decrease coverage by 2.65%.
The diff coverage is 26.66%.

@@            Coverage Diff             @@
##           master     #681      +/-   ##
==========================================
- Coverage   74.48%   71.83%   -2.66%     
==========================================
  Files          53       53              
  Lines        3422     3444      +22     
==========================================
- Hits         2549     2474      -75     
- Misses        873      970      +97     

Impacted Files	Coverage Δ
src/vorta/views/main_window.py	77.70% <6.66%> (-6.54%)	⬇️
src/vorta/views/misc_tab.py	81.39% <36.36%> (-15.67%)	⬇️
src/vorta/models.py	80.80% <75.00%> (-0.32%)	⬇️
src/vorta/keyring/darwin.py	0.00% <0.00%> (-100.00%)	⬇️
src/vorta/autostart.py	63.63% <0.00%> (-33.34%)	⬇️
src/vorta/network_status/darwin.py	33.89% <0.00%> (-27.12%)	⬇️
src/vorta/notifications.py	71.21% <0.00%> (-13.64%)	⬇️
src/vorta/keyring/abc.py	81.48% <0.00%> (-11.12%)	⬇️
src/vorta/network_status/abc.py	82.35% <0.00%> (-5.89%)	⬇️
src/vorta/config.py	94.44% <0.00%> (-5.56%)	⬇️
... and 3 more

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