Save pipeline connections in a consistent order

[SamCarlberg]

Fixes a problem with logical operations (and linked input sockets in general) having output connections deserialized prior to input connections, which lead to crashes when loading save files with operations that use linked input sockets.

Old save files using logical operations remain unusable, but creating a save file with those operations will guarantee a readable save file.

Fixes #919

[codecov-io]

Codecov Report

Merging #922 into master will increase coverage by 0.14%. The diff coverage is 70.14%.

@@            Coverage Diff             @@
##           master     #922      +/-   ##
==========================================
+ Coverage    54.5%   54.65%   +0.14%     
==========================================
  Files         304      305       +1     
  Lines        8318     8385      +67     
  Branches      535      540       +5     
==========================================
+ Hits         4534     4583      +49     
- Misses       3587     3605      +18     
  Partials      197      197