Avoid overwriting the first typeahead find letter

Grabbing the focus of the search entry caused the whole contents of the
entry to be selected. As this was done when entering a character to
bring up the search entry, it was easy to type a whole word, overwriting
the first (selected) character.

Fix the problem by setting the cursor position to the end of the search
entry immediately after grabbing focus.
Show placeholder when the events listbox is empty
Support case-insensitive search

If only lower-case characters are entered in the search field, use a
case-insensitive search. If there are any upper-case characters, use a
simple case-sensitive search.
Show only user events in the applications category

Refine the query for applications to only show events from the same user
that is running the application.
Use the system 12/24 hour clock setting

Use the clock-format key in org.gnome.desktop.interface to control
whether timestamps are shown in 12 hour or 24 hour format.

https://bugzilla.gnome.org/show_bug.cgi?id=710728
Make timestamp translatable