implementing history using GTK+

There has been feature that has been missing ever since Tracker's dedicated GUI search tool was written and I have now implemented it. The one major brickwall I had to overcome was understading the concepts of GUI design, specifically GtkComboBoxEntry and it's close relationship with GtkEntry, as well as GtkTreeModel and relatives.

Implementing this feature had me slouch through the docs for hours, and even led me to consult source code from a few sources and ended up implementing something similar to xfrun4, XFCE4's run dialogue. An extra nice thing I got from that dialogue is entry completion, which I implemented as well.

[update] The patch, on third try, has been committed!