**Is your feature request related to a problem? Please describe.**

Currently, the only way to show the About panel on macOS is by creating a `{role: 'about'}` menu item and click it. I need to be able to show it manually.

**Describe the solution you'd like**

Electron already has the method, so it's just about exposing this to JS and documenting it:

https://github.com/electron/electron/blob/8ba271efba691357ab1970c1a553b4e019125574/atom/browser/browser_mac.mm#L376

Would also be useful to be able to pass in the panel options directly to the method to override the defaults.

I would call the method: `app.showAboutPanel(options)`

**Describe alternatives you've considered**

None.
atom/browser/api/atom_api_app.cc
base::Bind(&Browser::IsDefaultProtocolClient, browser))
      .SetMethod("setAsDefaultProtocolClient",
                 base::Bind(&Browser::SetAsDefaultProtocolClient, browser))
      .SetMethod("removeAsDefaultProtocolClient",
                 base::Bind(&Browser::RemoveAsDefaultProtocolClient, browser))
      .SetMethod("setBadgeCount", base::Bind(&Browser::SetBadgeCount, browser))
      .SetMethod("getBadgeCount", base::Bind(&Browser::GetBadgeCount, browser))
      .SetMethod("getLoginItemSettings", &App::GetLoginItemSettings)
      .SetMethod("setLoginItemSettings",
                 base::Bind(&Browser::SetLoginItemSettings, browser))
#if defined(OS_MACOSX)
      .SetMethod("hide", base::Bind(&Browser::Hide, browser))
      .SetMethod("show", base::Bind(&Browser::Show, browser))
      .SetMethod("setUserActivity",
                 base::Bind(&Browser::SetUserActivity, browser))
      .SetMethod("getCurrentActivityType",
                 base::Bind(&Browser::GetCurrentActivityType, browser))
      .SetMethod("invalidateCurrentActivity",
                 base::Bind(&Browser::InvalidateCurrentActivity, browser))
      .SetMethod("updateCurrentActivity",
                 base::Bind(&Browser::UpdateCurrentActivity, browser))
      .SetMethod("setAboutPanelOptions",
                 base::Bind(&Browser::SetAboutPanelOptions, browser))
#endif
#if defined(OS_WIN)
      .SetMethod("setUserTasks", base::Bind(&Browser::SetUserTasks, browser))
      .SetMethod("getJumpListSettings", &App::GetJumpListSettings)
      .SetMethod("setJumpList", &App::SetJumpList)
#endif
#if defined(OS_LINUX)