Update LoginDialog to be type Qt::WindowModality

Currently the login window will cover other windows, in particular
the "welcome" window that pops up for new users.  This allows those
windows to be interacted with while still blocking interaction with the
main window.
Merge remote-tracking branch 'upstream/master' into 19623
Merge pull request #2765 from huffman/19623

Job #19623 - Update LoginDialog to be type Qt::WindowModality