Various improvements to event compression UI

The UI improvements as suggested by Allan
and Jakub are implemented in this commit:

* Use color theme suggested by Jakub for
  expanded rows.
* Use row separator to indicate boundaries
  of expanded header and it's group.
* Add hover effect to the expanded rows.
* Increase grid spacing and decrease label
  padding in popover grid.
* Use regular text everywhere in the detailed
  popover.
* Add padding for popover grid.
* Highlight the row pointed by detailed popover.

https://bugzilla.gnome.org/show_bug.cgi?id=709294