Share view/hidden keyboard fixes

Fixes issue reported in https://getsatisfaction.com/newsblur/topics/cant_share_on_ipad_with_hardware_keyboard.

The code depended on the show/hide keyboard notifications to animate the share view itself, and if the keyboard is split or hidden on iPad, these notifications don't get delivered, so the share view either gets stuck offscreen or onscreen, depending on the situation.

Also, the code ignored hide notifications when the share view was visible but not in the process of hiding itself, so you could end up with a share view hovering in the middle of the screen if you hid the keyboard while the share view was visible.