Make it easier to clear the formatting of tables

[ssddanbrown]

Our clear formatting option does work on tables, but only the cell contents where the cells are selected, which can leave akward styles left. It is possibly to clear all table formatting to reset it, but you have to know to drag-select from the block after the table to cover the whole table. This may be better as a table toolbar action.

Note: Ensure we retain table structure elements like merged cells.

[ssddanbrown]

Added via #4850