Handle or prevent invalid player IDs in scenarios

[historic_bruno]

It's possible to place units in Atlas for up to 8 players, but if the number of players is changed (either by manually editing the XML or with the player editor) then some units may have invalid player IDs. This causes errors when the scenario is loaded. Part of the problem can be addressed in Atlas itself, perhaps not saving units that are not assigned to a valid player. Or the player can be notified with a message "Player X has units assigned which will be lost. Continue?" It can also be solved by the map reader, e.g. not adding entities that have invalid player IDs.

Otherwise the game logic fails unpredictably.

[ben]

(In [9608]) Adds player editor to Atlas scenario editor. See #91 Fixes object panel so the list of players updates correctly. Atlas map settings are now observable and shared between various panels. Fixes panels for maps missing settings (caused JSON error) Changes map reader so that entities with invalid players cause a warning but are not added to the game. That prevents further errors. Fixes #869 Uses map size choices from simulation/data/map_sizes.json instead of multiple locations. Fixes #787 Prepares Atlas map settings for undo/redo support