Enhancement: line annotation display events

[deathaxe]

This commit makes some changes to the event handler, which is responsible to
call git blame and display line annotations.

The origin for the changes is issue #528. Even though the issue could not be
reproduced on Windows a couple possible enhancements were identified.

Fixes

1. Fail silently if the active row can't be identified because of an invalid
   or empty selection object returned by ST API.

Enhancements:

1. Hide line annotation if...
   a) multiple cursors are active
   b) selection is not empty
   c) caret is moved vertically in row

   As a result line annotation is displayed only if a single cursor is moved
   to a new unmodified line of text, if word wrap is disabled.

   This should help improving the editing experience and reduce the impact of
   some of the phantoms' drawbacks / limitations.

2. All checks are performed within the debounced code to avoid the line
   annotation to flicker up if text is selected via mouse.

[rwols]

Hi @deathaxe, I just checked out this branch and it makes the exception disappear. Nice work!