Support file-open check for multiple files

Support file-open check for multiple files

When dropping multiple files, make sure none of the files are already open.  If
all files were already open, then select the window and tab of the first file
that was open.  If at least one file was not already open, then simply ignore
the open files and open the unopened ones.

Also don't check if files exists inside application:openFiles: since files that
are passed here are assumed to already exists.  The existence check has been
moved to the system services message openFile:userData:error:.