Fix default User Template location

When we assign a string to the wxFilename, it takes the last section as
the filename (unless there is a trailing '/').  So on Linux, we got
/home/user for the Documents dir and /home/ was the directory while
'user' was the filename.  Appending directories 'kicad/templates' made
the final directory '/home/kicad/templates'.  Forcing this to be a
directory restores correct behavior.

(cherry picked from commit 0ee38653f7)

common/pgm_base.cpp

@@ -445,7 +445,7 @@ bool PGM_BASE::InitPgm()
     else
     {
         // Default user template path.
-        tmpFileName = wxStandardPaths::Get().GetDocumentsDir();
+        tmpFileName.AssignDir( wxStandardPaths::Get().GetDocumentsDir() );
         tmpFileName.AppendDir( "kicad" );
         tmpFileName.AppendDir( "template" );
         envVarItem.SetDefinedExternally( false );