Backport new default emulators to existing games in library

[Tony]

If you change a default emulator for a system, it only applies to games added in the future. There should be an option to update all games for that system to use the new default emulator when you change it.

[Brad Cheyne]

You use the bulk edit wizard to change games options. Click a platform on the left, then a game. Ctrl + A to highlight them all then right click -> Edit. Choose emulator then select the desired emulator from the drop down. The default emulator box is for importing, it's deigned this way for user control.