offscreen windows don't run their message loop

[mmu_man]

When creating a BBitmap that accepts view (like Pe does for its PText view), a BWindow is created that doesn't run a message loop.

This causes problem when going through all the windows of an application. While hey can probably be fixed with a timeout (cf. #15364), in some cases a timeout is not acceptable, like, for example a screen reader cannot accept being blocked for so long.