Created office finders for different OSes and a generic one, but they don't have any implementations yet.
Added preference screen to choose office path.

Currently having problems recursing correctly when looking in child
folders for soffice bin. Also, it loads OOo alright (when I also have
libreoffice installed), but it doesn't open the spreadsheet at all.
Don't require restart after changing soffice path. (Fixes #13)

... because the first phase is the one which connects, so if you
change the path before you start running, it will work. If you
change it half way through, the soffice bin will already be open.
In that later case, you wouldn't expect it to change.