Three patches for MathML

Hello,

I improved the way MathML constructs are inserted when the user have
already selected one part of a formula. The selected part is now cut and
paste into an empty placeholder of the new construct. This was already
the case for the basic Presentation MathML constructs using
TransformIntoType (though sometimes it does not work well) but now I
generalize this feature for every constructs. The changes are contained
in patch3.diff

Also, I fixed two bugs :  export of horizontal lines (patch1.diff) and 
floor/ceiling reversed (patch2.diff).

Fred