remove dc_contact_get_first_name() api

instead, dc_contact_get_display_name() should be used.

dc_contact_get_first_name() was created to save some space on the screen,
esp. on mobile devices,
however, this does not always work and has issues on its own
with some names ("Dr. Strangelove", ":) Name" and so on).

as with mailing lists, more apis with first_name() would be needed,
we decided to drop that instead of following that way.

it is also less an issue as some years ago as screens have become larger,
if really, needed, the ui can handle that more gracefully,
however, just using dc_contact_get_display_name() should be fine as well.

src/message.rs

@@ -1032,7 +1032,7 @@ impl Lot {
                         self.text1 = None;
                     }
                 } else if let Some(contact) = contact {
-                    self.text1 = Some(contact.get_first_name().into());
+                    self.text1 = Some(contact.get_display_name().into());
                 } else {
                     self.text1 = None;
                 }