[WIP] Table merge cells

[MathanM]

Added TableCell widget and option to merge cells using TableCell widget

[MathanM]

DavBfr I am able to successfully merge cells, but stuck on drawing borders. I recently started with flutter and don't have much experience with canvas. Appreciate your help!

[MathanM]

I think we cannot draw borders for the whole table instead we need to draw border for each cell based on its box.width and box.height. @DavBfr

[MathanM]

@DavBfr I am able to fully implement merge cells along with few additional features like repeat header whenever table in broken across pages, prevent only rendering of table headers instead switch to new page to render both header and body but they include breaking changes. Currently I have created a custom table by extending pdf/table. Let me know if you are still interested in the fix.