Introduce GcalEventPopover

GcalEventPopover gives a preview of the event, and handy access
to the event description and information. It also allows easily
joining meetings if any one if available.

The list of supported meeting URLs is small now, but I hope
more people will contribute as we go.

It is also possible to edit the event through an edit button.

src/theme/Adwaita.css

@@ -225,6 +225,25 @@ event.color-light:backdrop {
     color: rgba(0, 0, 0, 0.3);
 }
 
+/* Event Popover */
+popover.event-popover,
+popover.event-popover > box {
+  padding: 0;
+}
+
+popover.event-popover > box > button {
+  border-left-width: 0;
+  border-right-width: 0;
+  border-bottom-width: 0;
+  border-top-left-radius: 0;
+  border-top-right-radius: 0;
+}
+
+popover.event-popover textview.view,
+popover.event-popover textview.view text {
+  background-color: transparent;
+}
+
 .search-viewport {
   background-color: @theme_base_color;
 }