Quickfix for crashes when downloads start

Every new window added download_handle_start() as a signal handler to
the global WebKitWebContext. This is wrong anway, because the signal
handler sets the destination path; this should not be done multiple
times.

The actual crash happened when a window was closed and *then* a download
started. The window's signal handler still existed with a pointer to the
window's "struct Client". This struct, however, is now free'd and
invalid. Hence the crash.

To get rid of the crashes, only add the signal handler once. Sad thing
is, this makes the "status" level bar useless: It would only work for
the very first window ever created -- if it still existed. If that
window has been closed as well, we would still crash. Thus, remove
"status" completely.

We'll have to find a new way to announce the start of a download.

Closes #10.
Show the download manager when a download starts

This is kind of a quickfix as well (related to #10). I'll have to use
this for a while to see if I'm happy with it.