Feature: Merge cells in table

[20Attila03]

I noticed, that I am unable to merge cells which I would be able to do so using the website. I looked into the xml, it's a simple colspan or rowspan so non rectangular cells are not possible.

The extension is able to render it correctly.

Basically I would like you to add the following feature, based on what is available on draw.io-s site:

• [ ] If multiple unmerged cells are selected in arectangle add merge cells option.
  • [ ] colspan
  • [ ] rowspan
  • the two combined are possible
• [ ] If a merged cell is selected add unmerge cell option.