Keep target trigger index of the selection stored in session.
Fix trigger index on column change.
Make sheet corner black.
Change column header visuals.
Fix bad idea.
Fill trailing blank of the sheet header with canvas background colour.
Disable focus on sheet parent widget.
Merge pull request #238 from kagu/fix_target_trigger_index

Fix cursor position handling on column change
Merge pull request #240 from kagu/column_headers

Improve column header presentation