Fix bug 1527

Fix bug 1527
http://bugzilla.lyx.org/show_bug.cgi?id=1527

* src/mathed/*.cpp:
	- Track mode (math or text) when outputting latex code and
	  properly wrap commands that are in the wrong mode with
	  either \ensuremath or \lyxmathsym.

* src/mathed/MathParser.cpp:
	- Parse \ensuremath and \lyxmathsym, such that exported latex
	  produces the same result when imported back.
	- Replace latex commands with corresponding unicode symbols.

* src/Encoding.cpp:
	- Implement Encodings::fromLaTeXCommand() for converting either
	  a single or a sequence of latex commands to unicode symbols.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@25270 a592a061-630c-0410-9148-cb99ea01b6c8