style: Clean up dialogs stylesheet to fix layout issues

- remove unnecessary dialog type-specific padding, spacing and margin definitions that led to layout to problems
- bump the size of user avatar in authentication dialog
- nest css classes in the sass to better reflect the structure of dialog elements
- sync style of audio device selector with switcher-popup style
- sync text and padding styles with other elements
- fix incorrect button order in audio selection dialog

Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/7876
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3473>

js/ui/dialog.js

@@ -56,6 +56,7 @@ class Dialog extends St.Widget {
         this._dialog.add_child(this.contentLayout);
 
         this.buttonLayout = new St.Widget({
+            style_class: 'modal-dialog-button-box',
             layout_manager: new Clutter.BoxLayout({
                 spacing: 12,
                 homogeneous: true,
@@ -124,7 +125,7 @@ class Dialog extends St.Widget {
             keys = [];
 
         let button = new St.Button({
-            style_class: 'modal-dialog-linked-button',
+            style_class: 'modal-dialog-button',
             button_mask: St.ButtonMask.ONE | St.ButtonMask.THREE,
             reactive: true,
             can_focus: true,