Enhancements of data editor for tables

[DanyCaissy]

HTML Editing

We could add an HTML editor to allow users to modify the cell content, in this way they could add images or other things as to style their table accordingly.

The editor could look something like this: https://i.imgur.com/l5oRnFc.png

This is currently done in other solutions (such as tablepress) already.

Merging Cells

We could allow users to merge cells, if possible with the spreadsheet plugin.

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For these new options, we could add action buttons on top of the table which would apply to the current selection.