Summary of changes in this release
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* Switched to using SDL2.
* Gained support for full-screen stretching.
* Gained support for MacOS.
* Switched to using enet, replacing SDL_net.
* Audio code improvements.
* The multiplayer connection code has stablized.
* New game hotkeys.
* Migrate population across extended town networks.
* Added ability to migrate 10 people at a time across between towns.
* Gettext translation support.
* Several bug fixes.