Support checking of trigger existence in a column.
Add a delete selection button that does not delete selections.
Support retrieval of column from sheet manager.
Use sheet manager for retrieving the current column in delete selection button.
Support removal of triggers in the column model.
Remove selected trigger when delete button is clicked.
Signal update on selection removal.
Temporarily fix sheet update.
Check for absence of keyboard modifiers before moving the edit cursor in the sheet.
Fix delete selection button without selection.
Flush all sheet caches on selection removal.
Use sheet manager to retrieve the current column in the sheet.
Remove column caching from the sheet view.
Add simple column caching support to session.
Cache the latest retrieved up-to-date column.
Support removal of selection with the Delete key.
Implement trigger removal in the sheet manager.
Use sheet manager for removing selections.
Remove cache handling from the column object.
Merge pull request #185 from kagu/delete_selection

Support deleting selected trigger.