- Electron version: 1.3.6
- Operating system: Windows 7 x64
- Other: Microsoft Office 2013 (will make more sense in a bit)

`shell.openItem()` is a blocking call, it would be nice if it's async or has an async version to handle synchronous external application behavior.

Reproduce: 
Open a password-protected Microsoft Excel file, `shell.openItem()` will be blocking until the Password window is submitted or dismissed. Consequently Microsoft Word does not have this issue.

Related to #2761.

shell/common/platform_util_linux.cc
return XDGUtil({"xdg-open", path}, wait_for_exit);
}
bool XDGEmail(const std::string& email, const bool wait_for_exit) {
  return XDGUtil({"xdg-email", email}, wait_for_exit);
}
}  
namespace platform_util {
void ShowItemInFolder(const base::FilePath& full_path) {
  base::FilePath dir = full_path.DirName();
  if (!base::DirectoryExists(dir))
    return;
  XDGOpen(dir.value(), false);
}
bool OpenItem(const base::FilePath& full_path) {
  return XDGOpen(full_path.value(), false);
}
void OpenExternal(const GURL& url,
                  const OpenExternalOptions& options,
                  OpenExternalCallback callback) {
  
  
  if (url.SchemeIs("mailto"))
    std::move(callback).Run(XDGEmail(url.spec(), false) ? ""
                                                        : "Failed to open");
  else
    std::move(callback).Run(XDGOpen(url.spec(), false) ? "" : "Failed to open");
}
bool MoveItemToTrash(const base::FilePath& full_path, bool delete_on_fail) {