Remove obsolete focus setting code.
Set initial focus to the sheet.
Make delete button experimental.
Add keyboard backspace support.
Merge pull request #221 from kagu/fix_keyboard_focus

Fix keyboard focus
Merge pull request #230 from kagu/experimental_delete

Make delete button experimental
Merge pull request #231 from kagu/backspace

Add keyboard backspace support
Fix cursor position handling when changing column after trigger insertion.
Fix trigger index after insertion in the model side.
Fix target trigger index update after trigger insertion.
Fix style.