[TD] Fix win file spec backslash (fix #16646) (#16689)

* [TD]add method to clean win filespecs

- '\' in strings passed to Python as filespecs is interpreted as
  an escape of the following character.
- replace '\' with '/'

* [TD]remove '\' from filespecs before use

src/Mod/TechDraw/App/DrawUtil.h

@@ -277,6 +277,8 @@ public:
 
     static bool isWithinRange(double actualAngleIn, double targetAngleIn, double allowableError);
 
+    static std::string cleanFilespecBackslash(const std::string& filespec);
+
 
     //debugging routines
     static void dumpVertexes(const char* text, const TopoDS_Shape& s);