Summary:
New shortcuts for removing and adding rows.
For adding a row we now have Mod+(Shift)+Enter
For removing rows we now have Mod+Delete/Mod+Backspace
Before removing rows, the user is prompted to confirm, this prompt
can be dismissed and this setting can be remembered. User needs
to confirm only when using shortcut.
Old shortcuts are still active and shows information about this change.
This information is shown only once, after this shortcuts have default
behavior (zooming).
New users don't see this explanation.
Test Plan: Updated
Reviewers: georgegevoian
Reviewed By: georgegevoian
Differential Revision: https://phab.getgrist.com/D3655
