Remove project filter

[jeremypw]

The project filter is not really necessary since the number of projects open simultaneously is unlikely to warrant a search. The presence of the filter when the listbox is bound to a model gives rise to a run-time warning: GtkListBox with a model will ignore sort and filter functions (though the filter does in fact work).

It is debatable whether the scrolled window is needed either as about 12 projects can be shown without scrolling and many more before the popover exceeds the likely height of the window. However, it is left for now pending UX opinion.