[rekonq] ProgressBar solid color

[Lionel Chauvin]

> Firefox 4 adds progress bars on tabs.
> I am not sure it is a so good idea but it is an other way to remove the
> urlbar/progress bar/(status bar :D ?) confusion.

For 0.5.0 we should avoid invasive changes.

For 1.0 we could try another way;
Turn the background progress bar into a real progress bar widget, embedded to 
the right of the url bar, next to the rss icon.
This should solve unreadable text issues and ensure we follow Qt style.

Lionel