Slight command manager refactor

[kimden]

(so small that I make a separate PR for it)

I just wanted to restructure external commands but suddenly wished to combine a newly created AbstractResource class with existing Command class...

As a result I expect:

• Command implementations to not be member methods of CommandManager
• MapFileResource implemented (the original goal)
• Slight refactor with fresh eyes and more experience than in 2021 when I wrote that

(Regarding the first point, technically, 3 implementations depend on CommandManager, but the required things from it will later be moved to Context. The rest of them don't use CM, can be separated from it, but right now there's no need to move them elsewhere).

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[kimden]

Tested before the last posted commit (which doesn't change a lot), seems to work fine.