fix to load/save problems in Linux 

I was unable to open existing and save any point clouds with linux mint KDE. These issues were also reported in the forums by other users (using Kubuntu?). 

Using double semicolon ";;" as separator (i.e. not new line "\n") between the filters for the different file types in the QFileDialog call fixed the issues.  

I haven't been able to test this on Windows/Mac, but expect it should work fine on other platforms too as ";;" is the separator used in the official Qt docs.

-eero