Silent pics delayed trigger

[Marsu42]

This is one of Alex' own ideas, but I think it should be promoted to an official request to remind him once he's back :-) ...

... the idea is to continuously capture silent pics into the buffer, but *only* to save them after pressing halfshutter. This fixes the main problem with burst pics and fast action and the reason I don't use it for this - if the action starts, at least part of it be over once you managed to press the button.

The UI should show how much time at what fps is currently in the buffer so you can estimate when to save it - and/or there could be a menu option to save a fixed amount of xyz frames/time backlog after pressing halfshutter and from there on as much as the buffer holds.