Parameter to prevent cache flushing on deletion

Right now, calling view::delete always calls both cache_clear_all and menu_rebuild. That's perfectly ok to do in the UI, but when it hurts performance a lot when run in a batch: each delete will rebuild the menu, when one at the end of the batch, done by the caller, should be enough.

Attached patch (which should apply cleanly against 2.x-dev and with a small offset to 3.x-dev) adds a $clear parameter to view::delete, defaulting to TRUE. The situation should thus be the same for the outside world, but for example Features would be able to set it to false when mass reverting its default views, and call menu_rebuild after its loop.