This is a hassle-free BBA option intended for local play with multiple
Dolphin instances running *in the same system*. After selecting
**Broadband Adapter (IPC)** in the **SP1** slot in the GameCube section
in the settings, games that support LAN play will be able to discover
each other, without requiring third-party software or relatively complex
TAP setups.

The implementation is based on cpp-ipc, a high-performance inter-process
communication library that uses shared memory as transport layer.

Supported platforms are:

- [x] Linux
- [x] Windows
- [ ] macOS (cpp-ipc does not support this platform)
- [ ] FreeBSD (cpp-ipc does not support this platform)
- [ ] Android (cpp-ipc needs some adjustments; while it could work,
  launching two Dolphin instances within the same Android system may be
  both challenging and impractical)

Due to an oversight in our CMakeLists, pkg-config would attempt to find *minizip* 3.0.0 (which doesn't exist) instead of *minizip-ng* 3.0.0, or at least it was on my Manjaro Linux machine. This has been fixed. The new submodule is using minizip-ng 3.0.4, the same version that was being used before.

Adds the rcheevos library from RetroAchievements to the Dolphin Externals as a submodule. Change was verified to import correctly and build both via Visual Studio and via cmake ninja.

* moves dolphin-specific settings out of Base.props
* creates exports.props for externals, allowing to easily import
  individual Externals
* corrects some cruft that accumulated and probably contributed
  to msbuild overbuilding

maybe it makes the xml less scary :D