Some potential bugs spotted by llvm/clang

Some potential bugs spotted by llvm/clang

src/TextClass.h
src/insets/InsetTabular.h

  Overloaded virtual method missing the 'const' qualifier

src/insets/InsetCommandParams.h

  Missing constructor (breaks compilation with llvm/clang)

src/frontends/qt4/GuiWorkArea.cpp

  Missing parenthesis: `+' has priority over `?:' (I do not know
  whether this has a visible effect).

src/mathed/InsetMathFont.cpp

  Use of == instead of = in mathmlize()