MDL-44758 editor-atto: prevent table-in-table creation

Prevent new table creation, displaying table context menu instead when cursor is within caption, when multiple cells are selected, when selection contains both nodes within and outside of the table, and when selection contains nodes from multiple tables.
MDL-44832 behat: Fixed behat test
MDL-44747 editor_atto: Improved configuration error messages
weekly release 2.7dev
MDL-44758 editor-atto: prevent table-in-table creation

Prevent new table creation, displaying table context menu instead when cursor is within caption, when multiple cells are selected, when selection contains both nodes within and outside of the table, and when selection contains nodes from multiple tables.
MDL-44927 Atto: Improve table editing UX

Hide context menu items that don't apply.
Add "Edit table" menu entry to change the caption / headers.