event-popover: Rework UI and code

This builds on top of the existing UI/code, while adding a few improvements and
behavioral changes:

- The popover displays the changes-prevent icon when the even is read-only.
- Each section is properly separated with `GtkSeparator`s, with equal amount of
top and bottom margins.
- Location and Meetings section are mutually exclusive; only one is shown.
- When an event has no event note, the popover will always display that there's no
description.
- `GcalMeetingRow` has been ported to `GtkListBoxRow`.
- The action button adapts its icon and tooltip text depending on the event
permission.

src/gui/icons/icons.gresource.xml

@@ -6,5 +6,7 @@
     <file compressed="true">calendar-week-symbolic.svg</file>
     <file compressed="true">calendar-today-symbolic.svg</file>
     <file compressed="true">external-link-symbolic.svg</file>
+    <file compressed="true">edit-symbolic.svg</file>
+    <file compressed="true">info-outline-symbolic.svg</file>
   </gresource>
 </gresources>