[ATLAS] Map save / load rework

[brian]

We need to make the map saving/loading more mod friendly (per Philip's request). This means Atlas presents maps as files within mods, not arbitrary paths on the user's system. File extensions should be abstracted away, overwriting existing files should be handled nicely, etc.

Part of #1943.

[ben]

(In [10416]) Fixes Atlas map saving to force PMP extension on all OSes. See #889. Fixes Atlas map loading to more gracefully handle missing files. Atlas should no longer crash if a map was not found in VFS. Fixes #986.

[historic_bruno]

Duplicated by #631.

[ben]

In 13938:

Implements skirmish maps, based on patch by sanderd17, fixes #1198. Skirmish maps are like scenarios, except the player can choose their civ during match setup. To create a skirmish map: place some skirmish entities for each player in Atlas (see templates/skirmish/* for examples), uncheck the player's civ in Atlas' player panel if desired, and save in the maps/skirmishes directory. The map will appear in match setup under the "Skirmish" match type.
Implements custom, VFS-based map load/save dialogs for Atlas (replaces broken native file dialogs), fixes #631, #889.
Fixes map loading/saving to handle arbitrary subdirectories for better organization.
Adds default settings to Atlas player panel, fixes #1872. Each setting now has a checkbox to choose whether it should be saved with the map (avoids writing lots of useless default data for each map).
Adds map preview setting to Atlas, refs #1745.
Cleans up and simplifies some duplicate code.
Fixes optional serialization performance test.