Generate valid topics depending on world name

[chapulina]

Part of gazebosim/gz-sim#239 is making sure that all topics can handle entity names with spaces. This can be the case for the world.

This makes use of the AsValidTopic function to convert world names to valid ones whenever possible.

[codecov]

Codecov Report

Merging #163 (275f77b) into ign-gui3 (c80515d) will increase coverage by 0.00%.
The diff coverage is 61.53%.

@@            Coverage Diff            @@
##           ign-gui3     #163   +/-   ##
=========================================
  Coverage     60.31%   60.32%           
=========================================
  Files            20       20           
  Lines          2510     2523   +13     
=========================================
+ Hits           1514     1522    +8     
- Misses          996     1001    +5     

Impacted Files	Coverage Δ
src/plugins/world_stats/WorldStats.cc	94.59% <40.00%> (-2.58%)	⬇️
src/plugins/world_control/WorldControl.cc	79.20% <75.00%> (-0.29%)	⬇️

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[chapulina]

There's little value in getting this into Citadel because the string components serialization in Ignition Gazebo isn't working properly with whitespaces. This is fixed on Dome though, so I retargeted the PR on #164. We can backport that to Citadel in the future if we see the need.