Client API: get selected files

[86482t]

/manage_pages/get_page_info already exists, and it would be better if the not simple response also gave what files are currently selected in the gui, maybe return a list with the indexes of the selected files. Currently, what I use to do this is to send the keystrokes to trigger the hotkey that copies the selected hashes to the clipboard, but this would be a much nicer way to do it.

[bbappserver]

Related for short scripts just pass a list of hashes right to STDIN #270