Add FieldLoader support for Nullable<T>.
Allow maps to override options. Closes #3646.

Also set sensible defaults for most of the maps and mini games.
Reorganize map options panel.
Add "Build off Ally ConYards" option. Fixes #2464.
Remove dead InitialCash setting from PlayerReference.
Add starting cash option.
Allow a local support dir inside the game root. Fixes #3673.
Disable crates option for missions and mini games.
Tweak layout again.

C&C doesn't offer fragile alliances.
Merge pull request #3687 from pchote/mapoptions

Lobby map option improvements.
Merge pull request #3686 from pchote/portable-install

Support support dir inside the game root for portable installs.
separate diplomacy

into it's own container and yaml file
control IsVisible from IngameChrome
rewrite the Diplomacy panel

now with scrollbars, fixes #3443