[ATLAS] Add default options for player settings

[historic_bruno]

Currently in Atlas if you change one player setting, all the player data gets written to the scenario XML from defaults, which is annoying because they will override player_defaults.json. This is unintentional, so there should be default choices that let the map designer explicitly choose which settings to override from default.

See r13293 for why this is annoying.

[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.