ui: add initial Profile hover previews

The idea with these is that on notedeck you can just hover your cursor
over a profile link to see the profile. I just have a stub for now, but
full design coming soon after.

Also simplify the preview system even further with a macro. In the
future I imagine we can grep every preview in the codebase, and then
include this as a string inside this macro. This is some kind of
template metaprogramming insanity but in theory it could work.

Signed-off-by: William Casarin <jb55@jb55.com>

src/ui/username.rs

@@ -44,7 +44,7 @@ impl<'a> Widget for Username<'a> {
             };
 
             if let Some(profile) = self.profile {
-                if let Some(prof) = profile.record.profile() {
+                if let Some(prof) = profile.record().profile() {
                     if prof.display_name().is_some() && prof.display_name().unwrap() != "" {
                         ui_abbreviate_name(ui, prof.display_name().unwrap(), self.abbrev, color);
                     } else if let Some(name) = prof.name() {