Fix some bugs related to spaces in math:

Fix some bugs related to spaces in math:
- Interpret argument of LFUN_SPACE_INSERT correctly
- Use InsetMathSpace instead of InsetMathSpecialChar for "\ " (bug # 7728)
- Use InsetMathSpace instead of InsetMathChar for ~ (bug # 7728).
  This fixes also the display in LyX (previously a literal ~ was displayed).
Using InsetMathSpace enables also the "Insert Formatting" menu entries.
No file format change is needed, since the LaTeX export is unchanged.
Note that there are still some bugs related to spaces in math:
#7746, #7747, #7749, #7842

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@39947 a592a061-630c-0410-9148-cb99ea01b6c8