This fixes two issues related to keyboard help for DnDv2:

- If multiple DnDv2 blocks are on the same page, pressing global
  shortcut ("?") brings up multiple dialogs
- It's almost impossible to enter "?" into any of the text boxes in the
  block settings in Studio, since it brings up the keyboard help dialog
  instead

Not using "?" as a global shortcut for "Keyboard Help" also means that
DnDv2 will not conflict with other XBlocks and/or runtimes that might be
using the same shortcut to, e.g., bring up help dialogs.