style: Improve text scaling support

- new drawing function to convert px to em, for select instances
- updated fontsize function to convert fonts in pt to em
- replace instances of discrete sizes with defined values
- fix instances where assets or icons did not scale along with text
- rework panel buttons to accommodate the scaling padding and icons
- new 'scalable' definitions for elements that follow text scaling

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3033>

js/ui/dateMenu.js

@@ -729,7 +729,7 @@ const MessagesIndicator = GObject.registerClass(
 class MessagesIndicator extends St.Icon {
     _init() {
         super._init({
-            icon_size: 16,
+            style_class: 'messages-indicator',
             visible: false,
             y_expand: true,
             y_align: Clutter.ActorAlign.CENTER,