Now progress bar is implemented as custom widget on top of Fl_Widget
class. But FLTK already has Fl_Progress class for the same thing.
Progress bar looks the same with two exceptions:
1. No tooltip: it is not displayed for Fl_Progress even when set.
Normally progress bars, labels and other widgets for status
information don't have tooltips so I think it is ok.
2. When downloading file without Content-Length, progress bar is
always empty. With current implementation it changes for one percent
on every update and remains somewhere in the middle when download is
finished. For me keeping it at zero looks better.
Patch and python HTTP server for testing downloads without
Content-Length attached.
