transmission-remote priority settings should behave same as rpc spec

[Trina]

The rpc spec states the following regarding the arguments for the priority setting request methods:

 Just as an empty "ids" value is shorthand for "all ids", using an empty array
for "files-wanted", "files-unwanted", "priority-high", "priority-low", or
"priority-normal" is shorthand for saying "all files".

transmission-remote should also behave the same. It should be possible to set all the files for torrent to a priority level without needing to request the file ids first and specifying them all.