Don't block the TUI while refreshing feeds

[bacardi55]

At the moments, the TUI show a full screen message while refreshing feeds and thus stop the user to continue browsing tinylogs entries while refreshing.

This is done at the moment because otherwhise, the TUI would be frozen while refreshing. And I prefer having this message than an unresponsive TUI during the refresh phase (and at least the user know why).

It would be better if the refreshing mechanism doesn't block the TUI. It might need some heavy code change to move the timeline construction to an asynchronous tasks that won't block because of the way cview (the tui library) works (or at least who I understood it).