gedit-recent: don't require caller to manually lowercase filter string

gedit_recent_get_items filters out items from the recent file list that
match a set filter string. The filter string must be explicitly
lowercased by the caller, which is confusing.

This commit changes the code to allow the caller to pass the filter
string unmodified. gedit_recent_get_items will now implicitly lowercase
the string on behalf of the caller.

https://bugzilla.gnome.org/show_bug.cgi?id=753481