OS additions and fixes for WebAssembly/asm.js

- Implement alert, shell_open, set_window_title
 - Add locale lookup, fixes #2477
 - Print without color control sequences
 - Move get_executable_path implementation to OS_JavaScript

drivers/unix/os_unix.cpp

@@ -522,9 +522,6 @@ String OS_Unix::get_executable_path() const {
 	delete[] resolved_path;
 
 	return path;
-#elif defined(EMSCRIPTEN)
-	// We return nothing
-	return String();
 #else
 	ERR_PRINT("Warning, don't know how to obtain executable path on this OS! Please override this function properly.");
 	return OS::get_executable_path();