[Dialogs] OK button does not have not focus when tabbing to button bar of TextInputDialog on Mac and Linux

This is similar to the issue with RT-39491 but for TextInputDialog rather than Alert.

To reproduce, run the following on Mac or Linux:

1) Run HelloAlert
2) Click on the "Name" button (or hit tab 3 times then press the space bar) to bring up the dialog
3) Press TAB to move focus to the button bar
4) BUG: The Cancel button has focus even though the OK button is selected