Resolve Single Player's Missions grouping bug, affecting mods/Windows users.

[SoScared]

Currently mods based off OpenRA's bleed don't have access to mission groupings, possibly only affecting Windows users.

Relevant OpenRA PR: OpenRA/OpenRA#14868 and issue: OpenRA/OpenRA#11599

This issue is fixed on OpenRA's bleed but seemingly still relevant with mods updated with the bleed engine. As mentioned in the referenced PR this still could be a Windows issue.

Example1 uRA, engine version: (20180318).

The uRA master version contains the mission entries as examples as shown above:
https://github.com/RAunplugged/uRA/blob/master/mods/ura/missions.yaml
https://github.com/RAunplugged/uRA/blob/master/mods/ura/maps/missions/intervention/map.yaml
https://github.com/RAunplugged/uRA/blob/master/mods/ura/mod.yaml#L194

Example2 Generals Alpha, engine version: gen-20180323. Altered for use as example.

In both examples above, the mission.yaml defines some of the maps' directory with 'Group A' however all maps are thrown into the default catch-all group 'Missions'.

[SoScared]

Added mission-selector examples to the master repository: 37d9121

[SoScared]

This issue resolves itself when playing the game through travis built installers.