New function `IsBlockedByModal` and new flag `ImGuiInputFlags_RouteUnlessModal`

[rodrigorc]

This is my draft for implementing #7643.

I'm aware that modal windows are not a priority, because they are seldom useful in most ImGui use cases. But I happen to need them, so I'm trying to push them a little bit.

I'm not sure that is the proper way to implement it, that routing thing is still a bit of a mystery to me, but it seems to work just fine.

I'll be happy to receive any correction or feedback.