Add cfg option for opening files outside terminal

Since adding the "open file from transmission-remote-cli" feature, there
has been a commit which made this feature a bit annoying for me since
opening a file would've meant that transmission-remote-cli should stop
until the process running using the selected file would have ended.

Therefore, if 'file_open_in_terminal' is set to False, 'xdg-open <file>'
will start in the background, independent from the terminal, with stderr
and stdout redirected to /dev/null. The 'transmission-remote-cli'
process will not be stopped. By default, this option is disabled (set to
True).
Search file in other locations before opening it

Since some users prefer to save incomplete files in a separate directory
('incomplete-dir-enabled' set to true), this will make sure to check
that directory as well. In addition, it will also append '.part' to the
filename if the user has the 'rename-partial-files' enabled in
transmission.

No check is made if those options are actually enabled, because I
thought it would add unneeded complexity. I hope nothing bad will
happen.