Add source position tracking for grid tables

[messani]

Fixes #881

Adds source position tracking for grid tables and corresponding tests.

[xoofx]

Suggested change

	ResetLine(newLine, 0);
	ResetLine(newLine, column);

Shouldn't it pass column?

[messani]

Sorry about that — the first fix for this issue was a bit of a hack, and part of that code was left behind. It shouldn't have affected functionality, but it wasn't clean. I've corrected it now, and it should be fine. Thanks for spotting it!

[messani]

Since I'm not the original author, my contribution might not be entirely ideal.
The original ProcessLine method works the same way as before.
I've introduced a new method, ProcessLinePart, which doesn't add an entry to the LineStartIndexes variable and doesn't increment LineIndex.
Unfortunately, I'm not entirely sure how the BlockProcessor is designed internally, so I hope this change doesn't cause any issues.

[xoofx]

Thanks for this. As I have been quite away from the details on this project, I will have to double check things, but it looks ok. 🙂