gui/fading-label: Rewrite using PangoLayout directly

Instead of an internal label widget, store the text and the PangoLayout
directly, and only render them. Cache as much as possible to avoid
font reshaping.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/765>

src/gui/gcal-event-widget.blp

@@ -54,7 +54,7 @@ template $GcalEventWidget: Widget {
       }
 
       Inscription {
-        text: bind summary_label.label;
+        text: bind summary_label.text;
         hexpand: true;
         vexpand: true;
         xalign: 0.0;
@@ -68,13 +68,13 @@ template $GcalEventWidget: Widget {
       spacing: 4;
 
       $GcalFadingLabel summary_label {
-        label: bind template.event as <$GcalEvent>.summary;
+        text: bind template.event as <$GcalEvent>.summary;
+        hexpand: true;
+        vexpand: true;
       }
 
       Label timestamp_label {
         halign: end;
-        hexpand: true;
-        vexpand: true;
 
         styles [
           "dimmed",