Set pwd on save

I do loads of file I/O with maxima.
When using wxMaxima to do it when I create a new file doing file I/O (besides working around issue #360 which this patch depends on) I currently have to do the following before the file can be found:
 -save the .wxm-file (which is logical: I expect maxima to read files relative to the path my .wxm file is in. If I haven't saved my maxima session yet it has not yet been assigned a name or a directory => how should it know where the files it has to read are located?)
 - close wxMaxima
 - open the file again, since that triggers maxima's pwd to be set.

The current patch (which naturally has to be based on Patch #360) will set maxima's PWD on saving a file, too making the closing and opening the .wxm file to set maxima's PWD unneccessary.