this patch makes a couple different changes:

* previously the grab/ungrab was happening inside of
  selection_classic::{create,destroy}. this meant that
  `--freeze` was ineffective when edge mode was used.
  the patch moves those calls outside of it, so that `--freeze`
  can work on edge mode as well.
* because the grab/ungrab was happening inside selection
  destroy, there were multiple races. one of them was that the
  server would be ungrabbed after selection was done but
  *before* the shot was captured. the sleep introduced in commit
  93e475d widened the race window and made it more easily
  noticeable. the patch makes it so that we don't ungrab the
  server until the shot is actually taken.

Fixes: resurrecting-open-source-projects#381