Coding style adjustments
Update year & URL in about window
Remove option "sane sort indicator"

One shall use GTK3 option gtk-alternative-sort-arrows instead.
configure.ac: Use silent rules
Remove deprecated call
Loading packages is now done in a separate thread

So GUI isn't blocked, and even if things aren't actually faster (though they
might be a bit) they'll probably feel it.

Note: As a side effect, show_error() isn't blocking anymore.
Bump to 1.2.1; Update HISTORY