vis: perform undo snapshotting more rarerly

Do not take snapshots after every operation in insert/replace mode.
As an example up until now we would take a snapshot after every
<Backspace>/<Delete> press, hence when undoing changes each character
would be restored individually. The same applies for <C-w> and related
actions.

From now on we only snaphost when:

 - transitioning from insert/replace mode to normal mode (but not when
   switching to operator pending mode)

 - an operation takes place from normal mode

 - an idle time expires in normal/replace mode

0 files changed, 0 insertions, 0 deletions