Add #PB_Shortcut_TabNavigation flag to ShortcutGadget()

[Quin]

Currently, the ShortcutGadget() captures tab and shift+tab as valid keyboard shortcuts, making it very difficult for a screen reader user to get out of a ShortcutGadget if they happen to land on one. You have to use advanced screen reader commands that not everyone knows about and it's really janky. For this reason, I propose adding a Flags parameter to ShortcutGadget(), with the #PB_Shortcut_TabNavigation flag. It's function will be very similar to #PB_Editor_TabNavigation for the EditorGadget(), but handling both tab and shift+tab.

[nsstudios]

Omg yes!
+1
This would make using shortcut fields like IDE's own a looot easier.