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
Add GlUtilClockFormat to gl-enums
Add GlEventViewRow

Add a dedicated object for listbox rows.
Use GlEventViewRow in GlEventView

Adapt the GlEventView code to use the result from the row rather than
querying the journal again using the cursor.
Only load logs for the current category

Rather than loading events for every available category, only load and
show log events for the category which is currently selected.