Parse \multicolumn in math (bug 396)

Parse \multicolumn in math (bug 396)

The math parser could not handle multicolumn grids. This is a problem because
there is no true ERT in math (everything is parsed).
Now multicolumn cells are parsed correctly. The display is also somewhat OK,
but apart from that any multicolumn related UI is missing. Since the file
format change is now done the UI can be added at any later point. The most
important part of bug 396 is now fixed: tex2lyx does not create invalid .lyx
files anymore for formulas containing \multicolumn.

I updated the tex2lyx test cases that produce correct output. tex2lyx does
still produce invalid output for the test cases which are not updated because
of the previous format change.