Overhaul handling of font families

* Specify font families in the theme.
* Load fonts eagerly when loading themes, instead of loading
  them lazily when rendering.

Co-Authored-By: Antonio Scandurra <me@as-cii.com>
Co-Authored-By: Nathan Sobo <nathan@zed.dev>

zed/src/workspace/pane.rs

@@ -208,14 +208,12 @@ impl Pane {
                                     Align::new(
                                         Label::new(
                                             title,
-                                            settings.ui_font_family,
-                                            settings.ui_font_size,
+                                            if is_active {
+                                                theme.workspace.active_tab.label.clone()
+                                            } else {
+                                                theme.workspace.tab.label.clone()
+                                            },
                                         )
-                                        .with_style(if is_active {
-                                            &theme.workspace.active_tab.label
-                                        } else {
-                                            &theme.workspace.tab.label
-                                        })
                                         .boxed(),
                                     )
                                     .boxed(),