message-panel: add needs-attention when a new message arrives

Because when a build fail without seeing an error (for example if you don't have make or meson in a project that need it) this is useful to have, to see that something (and particularly, what) went wrong.

Probably fixes #111 (closed).

I have noticed something however. The needs-attention dot doesn't seems to look the same in the "Problems with build" on the left pane (where errors and build progress steps are showed) and in the "message" on the bottom pane (what I've added).

And I find the first one better looking.