Clean up context-specific options

The big conditional near the top of the file was full of stuff that was
commented out because it didn't work or was set on a different OS or
environment and didn't work in the current one.

Using more fine grained conditionals I've been able to re-enable all
options so that they only run in the contexts they're supposed to, like
choosing the right font for Windows™ or the right listchars for TTY
terminals. As a nice side-effect, PowerLine is used in all terminals.
Set buffers to hide on close, instead of being deleted

I forgot what the problem was with this before but I think it was
latex-suite related and it's useful to me now, so I'm, re-enabling it
and let's see what happens.
Indent with spaces instead of tabs
Add speeddating format used in PO files

The PO internationalisation files in screenfreeze's messagingmenu
project use a format SpeedDating doesn't recognise, so I added it to run
after SpeedDating is loaded.