Add map preview support in Atlas

[historic_bruno]

We now have map preview images displayed during match setup. Currently, to add a preview requires manual work on the part of the artist/designer, to create a properly sized image, move it to the correct location, and paste the filename into the map's XML (for scenarios) or JSON (for random maps).

The simplest enhancement to Atlas would be a new field in Atlas' map settings tab. A more advanced feature would be adding a map preview tool into Atlas, which would automatically size and save a preview image (probably the current camera view, like a screenshot) and update the map data. This would have the bonus of making map previews much easier to update.

Edited (24/Apr/22)

from ticket #6514
It would be a great feature to have a button in Atlas that generates a preview image for the map being worked on. That way all preview images can be standardized and anyone making a new map can easily get a preview image for their map without needing to muck around with screenshots and image editing software.

Features and Behavior:

1. A small button in the map setting panel under the Preview field.
2. Clicking this button generates an image of the whole map in the proper resolution and format and places the image in the correct game directory.
3. Also doing this automatically enters the file name of the new image in the Preview field.
4. Bonus: Add a "Browse..." button as well that opens the map preview folder and allows the player to manually select a preview image.

[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.

[Langbart]

#6514 closed as a duplicate of this ticket