Implement OS.get_process_ID for Windows

Based on code by @ratsdiov.
Closes #1733

platform/windows/os_windows.cpp

@@ -55,6 +55,7 @@
 
 #include "shlobj.h"
 #include <regstr.h>
+#include <process.h>
 
 static const WORD MAX_CONSOLE_LINES = 1500;
 
@@ -1884,6 +1885,10 @@ Error OS_Windows::kill(const ProcessID& p_pid) {
 	return ret != 0?OK:FAILED;
 };
 
+int OS_Windows::get_process_ID() const {
+	return _getpid();
+}
+
 Error OS_Windows::set_cwd(const String& p_cwd) {
 
 	if (_wchdir(p_cwd.c_str())!=0)

platform/windows/os_windows.h

@@ -245,6 +245,7 @@ public:
 
 	virtual Error execute(const String& p_path, const List<String>& p_arguments,bool p_blocking,ProcessID *r_child_id=NULL,String* r_pipe=NULL,int *r_exitcode=NULL);
 	virtual Error kill(const ProcessID& p_pid);
+	virtual int get_process_ID() const;
 
 	virtual bool has_environment(const String& p_var) const;
 	virtual String get_environment(const String& p_var) const;