Show properly on screen "indented" maths.

Show properly on screen "indented" maths.

This is done by two things:
1/ the equation returns LefAlign as display() value

2/ Inset::indent() return a value (in general 0) that should be added
   on the left (or right in rtl) of the inset when it is flushed.

The code that uses these values is in TextMetrics::computeRowMetrics.