[MixMaster] Fix crash when using Tcl 8.6

Tcl/Tk changed some operations which previously resulted in undefined behavior
into errors, including one case that occurred when updating the "Mixtures" menu
in MixMaster.

MixMaster now modifies the existing Menu when adding an item, rather than
completely regenerating it, which avoids this problem.

Cherry-pick of trunk r3162.

git-svn-id: https://cantera.googlecode.com/svn/cantera/branches/2.1@3163 02a645c2-efd0-11dd-984d-ab748d24aa7e