Add ui_combo_box_add_to_history() and use to prevent blocks of
duplicates (but not all duplicates) from search history combo boxes,
and limit history to 30 entries.


git-svn-id: https://geany.svn.sourceforge.net/svnroot/geany/trunk@999 ea778897-0a13-0410-b9d1-a72fbfd435f5