Provide progress indicator for searches.

Searches can be slow, so users need an indicator.  The indicator is
also used when opening a message list or when refreshing it, or when
loading more messages.

This also removes some unused code.
Remove the progress bar from the message list footer.

More often than not, it's scrolled out of view.
Other progress indicators have been implemented that
are always in view.
Provide a progress indicator while loading remote search results.
Merge remote-tracking branch 'zjw/progress_indicators'

* zjw/progress_indicators:
  Provide a progress indicator while loading remote search results.
  Remove the progress bar from the message list footer.
  Provide progress indicator for searches.

Conflicts:
	src/com/fsck/k9/activity/MessageList.java