Allow a command within a dialog to be repeated without closing said dialog

[eugenesvk]

I'd like to seriously overload the default Space dialog ("Space mode")
For example, I'd like to move all the "View mode" commands there to save one key
But I'd love to be able to save two keys or even more! For this I'd need to be able to make some commands (e.g., page_down,page_up) not to close the dialog when they're invoked, so the "Space mode" becomes "partially sticky" — some non-repetable commands execute once and close the dialog, while some repeatable commands keep it open until I hit Space again

Then, instead of holding a hard-to-reach-by-default Control and pressing b/f to PageDown/Up you could just press Space and release it and do the same — much easier!

Lowering the cancellation barrier I wouldn't even need to think much in advance whether I want to invoke a "sticky" version or the non-sticky one (although I could have that as well in the same dialog!)