We provide some sort of "type-ahead" search in the "Download an OS"
page, where users can just type any letter in their keyboard and that
will perform a search in the available OSes for Download.

When a new key is pressed, the key-press-event handler was constantly
calling downloads_search.grab_focus (), which makes the GtkSearchEntry
select its text, causing any following key presses to overwrite the
existing ones.

The solution to this issue is to only grab_focus () the search
entry when it doesn't have focus yet. In other words:

if (!downloads_search.has_focus) {
    downloads_search.grab_focus ();
}

Fixes #357