Broadcast improvements for drag/drop and bracketed paste

Problems today:

- Drag/drop requires the "StringSent" event handler to read text that was sent into one control to put it into the others.
- Drag/drop path translation broadcasts the same input for all panes even if their path translation styles differ
- Paste displays a warning dialog if the *active control* isn't using bracketed paste; this means that a broadcast group containing one BP and one non-BP control will display NO warning

see also #16287