Major plugin refactor and cleanup.

Switched to POCO library for unified platform/library interface.
Deprecated the external module API. It was creating more problems than solving.
Removed most built-in libraries in favor of system libraries for easier maintenance.
Cleaned and secured code with help from static analyzers.

vendor/POCO/cmake/test_compiler.cpp

@@ -0,0 +1,9 @@
+#include <iostream>
+#include <string>
+
+int main()
+{
+  std::string str = "Try to compile";
+  std::cout << str << '\n';
+  return 0;
+}