Re-enable buttons on cancel

[samuel-w]

Solves this problem: #332 (comment)

[codecov-io]

Codecov Report

Merging #772 (1d6b9e5) into master (6493e17) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #772      +/-   ##
==========================================
+ Coverage   73.73%   73.76%   +0.02%     
==========================================
  Files          53       53              
  Lines        3587     3591       +4     
==========================================
+ Hits         2645     2649       +4     
  Misses        942      942              

Impacted Files	Coverage Δ
src/vorta/views/archive_tab.py	84.65% <100.00%> (+0.11%)	⬆️
src/vorta/views/main_window.py	77.84% <100.00%> (+0.14%)	⬆️

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[m3nu]

Thanks. 🤩

For 0.7.2 let's focus on stability. Then more risky PRs after the release.