Fix crash on simulation loading caused by missing signal actions list

[orwell96]

As title says, this fixes a crash when loading a previously saved simulation
It appears to happen as a result of the signal actions not being saved in the JSON file.

OT: You are probably no longer maintaining and continuing this project. I don't expect you to actually review and merge this.
As I am actually pretty impressed by the work you put into this project, and am sad to see this lie around unfinished, I have decided to fork and continue it, although starting off from 0.6. If you're interested: https://github.com/orwell96/ts2-fork

[codecov]

Codecov Report

Merging #49 (7d786e4) into master (97df100) will decrease coverage by 2.49%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master      #49      +/-   ##
==========================================
- Coverage   76.26%   73.77%   -2.50%     
==========================================
  Files          38       38              
  Lines        2600     2208     -392     
==========================================
- Hits         1983     1629     -354     
+ Misses        494      459      -35     
+ Partials      123      120       -3     

Impacted Files	Coverage Δ
simulation/trains.go	55.91% <0.00%> (-4.96%)	⬇️
server/requests.go	75.00% <0.00%> (-6.82%)	⬇️
simulation/color.go	43.47% <0.00%> (-6.53%)	⬇️
plugins/routes/routes.go	70.00% <0.00%> (-6.00%)	⬇️
simulation/signal_conditions.go	58.18% <0.00%> (-5.09%)	⬇️
plugins/points/points.go	64.28% <0.00%> (-4.14%)	⬇️
simulation/track_signals.go	77.71% <0.00%> (-3.94%)	⬇️
server/websocket.go	75.00% <0.00%> (-3.58%)	⬇️
server/connection.go	73.58% <0.00%> (-3.09%)	⬇️
... and 29 more

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