event-editor/notes-section: Follow AdwEntryRow's style

The current notes text view has had a 2px offset for
an unknown amount of time, due to us magic-numbering
margins and/or padding. This approach works a little
bit differently, in that it uses the `.editable-area`
style class and sets the overlay's CSS name to "box"
to use a predictable size of margins from AdwEntryRow.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/738>

src/gui/event-editor/gcal-notes-section.blp

@@ -2,42 +2,60 @@ using Gtk 4.0;
 using Adw 1;
 
 template $GcalNotesSection: Adw.PreferencesRow {
-  activatable: false;
   focusable: false;
+  selectable: false;
 
   styles [
     "entry",
-    "notes-section",
   ]
 
+  GestureClick {
+    pressed => $on_row_pressed(notes_text) not-swapped;
+  }
+
   child: Overlay {
+    css-name: "box";
+
+    styles [
+      "header",
+    ]
+
     [overlay]
-    Label placeholder {
-      label: _("Notes");
+    Adw.Bin {
       can-focus: false;
       can-target: false;
-      xalign: 0.0;
-      yalign: 0.0;
-      visible: bind $empty_string(notes_text.buffer.text);
 
       styles [
-        "dimmed",
-        "placeholder",
+        "editable-area",
       ]
+
+      child: Label placeholder {
+        label: _("Notes");
+        xalign: 0.0;
+        yalign: 0.0;
+        visible: bind $empty_string(notes_text.buffer.text);
+        margin-top: 12;
+        margin-bottom: 12;
+
+        styles [
+          "dimmed",
+          "placeholder",
+        ]
+      };
     }
 
     child: Adw.Bin {
-      css-name: "box";
+      styles [
+        "editable-area",
+      ]
 
       child: TextView notes_text {
         hexpand: true;
-        top-margin: 6;
-        bottom-margin: 6;
-        left-margin: 6;
-        right-margin: 6;
         sensitive: true;
         wrap-mode: word_char;
         accepts-tab: false;
+        top-margin: 12;
+        bottom-margin: 12;
         state-flags-changed => $on_notes_text_state_flags_changed_cb(template) not-swapped;
 
         accessibility {