[winforms-dotnet5] Move content - Dialogs and popups

Related to dotnet/docs-desktop#72

When porting and migrating source content (presented as a [link]() that goes to an article), open the source material up. Find out where it exists in the TOC. In the source project #72 navigate down to the **.NET Framework 4 TOC and my notes** collapsible region and expand it. Find the article title in the TOC and review the notes.

- [ ] Dialogs and popups\
This content should mirror what is in WPF. This content doesn't really exist in the .NET Framework winforms area
- [ ] Overview ([link](https://docs.microsoft.com/dotnet/desktop/winforms/how-to-display-dialog-boxes-for-windows-forms?view=netframeworkdesktop-4.8) [link (WPF)](https://docs.microsoft.com/dotnet/desktop/wpf/windows/dialog-boxes-overview?view=netdesktop-5.0))
- [ ] Common tasks
- [ ] Display a message box ([link (WPF)](https://docs.microsoft.com/dotnet/desktop/wpf/windows/how-to-open-message-box?view=netdesktop-5.0))
- [ ] Open a form ([link (WPF)](https://docs.microsoft.com/dotnet/desktop/wpf/windows/how-to-open-window-dialog-box?view=netdesktop-5.0))
- [ ] Close a form ([link (WPF)](https://docs.microsoft.com/dotnet/desktop/wpf/windows/how-to-close-window-dialog-box?view=netdesktop-5.0))
- [ ] Display a system dialog box ([link (WPF)](https://docs.microsoft.com/dotnet/desktop/wpf/windows/how-to-open-common-system-dialog-box?view=netdesktop-5.0) [link](https://docs.microsoft.com/en-us/dotnet/desktop/winforms/advanced/how-to-capture-user-input-from-a-printdialog-at-run-time?view=netframeworkdesktop-4.8))
- [ ] Get or set the main form ([link (WPF)](https://docs.microsoft.com/dotnet/desktop/wpf/windows/how-to-get-set-main-application-window?view=netdesktop-5.0))