make paste focuses the pasted cell

This is to make paste behave in the same manner that insert cell above
and below currently work (inserting a new item causes that item to
receive focus).

This solves a problem where, if a user pastes below a cell whose bottom
edge is below the current viewing area, they have no indication that any
action has occurred. The same applies for paste above a cell whose top
edge is occluded.
Merge pull request #4974 from ivanov/better-paste

make paste focus the pasted cell