[SHELL32][EXPLORER] Update system-wide environment variables (#663)

- Implement shell32 RegenerateUserEnvironment() function and use it.
- CShellBrowser and CDesktopBrowser implements WM_SETTINGCHANGE actions.

CORE-1459, CORE-14397

sdk/include/reactos/undocshell.h

@@ -526,6 +526,9 @@ typedef struct
  * Misc Stuff
  */
 
+BOOL WINAPI
+RegenerateUserEnvironment(LPVOID *lpEnvironment, BOOL bUpdateSelf);
+
 /* SHWaitForFileToOpen flags */
 #define SHWFF_ADD     0x01
 #define SHWFF_REMOVE  0x02