Generate valid topics depending on world name

[chapulina]

Redo of #163 targeting Dome.

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 #164 (da16923) into ign-gui4 (ce4bf4f) will decrease coverage by 0.00%.
The diff coverage is 61.53%.

@@             Coverage Diff              @@
##           ign-gui4     #164      +/-   ##
============================================
- Coverage     61.57%   61.57%   -0.01%     
============================================
  Files            21       21              
  Lines          2790     2803      +13     
============================================
+ Hits           1718     1726       +8     
- Misses         1072     1077       +5     

Impacted Files	Coverage Δ
src/plugins/world_stats/WorldStats.cc	94.78% <40.00%> (-2.50%)	⬇️
src/plugins/world_control/WorldControl.cc	79.20% <75.00%> (-0.29%)	⬇️

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[JShep1]

LGTM with all tests passing