Allow showing all log events for the current boot

If -1 is passed as the requested number of results for a GlJournalQuery,
return a list of all results that match since the last boot.
Show messages from current boot for all categories

https://bugzilla.gnome.org/show_bug.cgi?id=709297
Add dim-label style class to no results icon

https://bugzilla.gnome.org/show_bug.cgi?id=711209
Make GlJournalResult a ref-counted boxed type