fix(appearance): Multiple fixes + refactor (#1244)

- Switching theme mode is now reflected immediately in the settings application;
- Color changes are propagated immediately when selected;
- Imported theme is now loaded immediately;
- Interface density is now responsive when selected;
- Font Selection is updated to the screen when changes are committed;
- Icons toolkit's "Apply to Gnome" toggle is now responsive;

Logical fixes:

- Style (Round, Square, etc) is now sync'ed between dark & light mode;
- Interface Density is now updated for both modes;
- Window Management is also updated for both modes;

Improvements/Perf fixes:

- Removed code paths where the same configuration field was written multiple times in the same pass;
- Stopped completely rebuilding the whole page when theme needed to be rebuild;
- Simplified initialization of the Appearance module;
- Extracted theme manipulation to its own module for future performance improvement;

---------

Co-authored-by: Michael Aaron Murphy <[email protected]>

Author: 8roken

cosmic-settings/src/app.rs

@@ -756,8 +756,9 @@ impl cosmic::Application for SettingsApp {
                 }
             }
 
-            Message::SetTheme(t) => return cosmic::command::set_theme(t),
-
+            Message::SetTheme(t) => {
+                return cosmic::command::set_theme(t);
+            }
             Message::OpenContextDrawer(page) => {
                 self.core.window.show_context = true;
                 self.active_context_page = Some(page);