Rectable selection fixes

[reejk]

There is two changes related to rectangular selection with zero width.

1. When pressing Tab, tabulation inserts at selection column like there is multiple cursors #397

2. When pressing Backspace, avoids cursor jumping up and losing lines from selection in result.

PR #425 is also about this problem.

How to reproduce: Replace the text in the Sample with the text below. Select all the semicolons and press Backspace twice. Selection will lose the last line.

Regular text
	{
		Some block
	}
	
void test();
void test();
void test();
void test();
void test();

The root cause is the precision of mathematical calculations. The HeightTree.GetVisualPositionFromNode method returns a number slightly smaller than what TextView.GetVisualLineFromVisualTop expects for the same line.
The totalHeight sum loses precision, and this totalHeight is also the sum of other calculations. The more work you do with the editor, the lower the precision of these calculations. At some point, the rectangular selection may lose all the initially selected lines.