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/edit-symbolic.svg

@@ -0,0 +1,4 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<svg height="16px" viewBox="0 0 16 16" width="16px" xmlns="http://www.w3.org/2000/svg">
+    <path d="m 12.277344 0.832031 c -0.578125 0.007813 -1.167969 0.230469 -1.691406 0.753907 l -9 9 c -0.375 0.375 -0.585938 0.882812 -0.585938 1.414062 v 3 h 3 c 0.53125 0 1.039062 -0.210938 1.414062 -0.585938 l 9 -9 c 1.789063 -1.789062 0.082032 -4.390624 -1.890624 -4.570312 c -0.082032 -0.011719 -0.164063 -0.011719 -0.246094 -0.011719 z m -1.777344 3.605469 l 1.0625 1.0625 l -7.0625 7.0625 l -1.0625 -1.0625 z m 0 0" fill="#222222"/>
+</svg>