Apply the modification at the cursor position to each selected line
rather than just setting all selected lines to the active line's text.
Still doesn't have good results in any nontrivial cases, but it's at
least sometimes useful.
ptr_vector hasn't been updated for C++11, so despite being specifically
designed to store pointers to objects it's less safe and not really any
easier to use than a regular vector of unique_ptrs
iterator_to requires that the object be in the list, which is sometimes
not the case when processing a commit which removed the active line or a
selected line. To handle this, add a checked version that returns
Events.end() when it is not in the list rather than crashing.
