Draft: Test of potential fix for TreeView glitch

Before calling gtk_tree_view_scroll_to_cell(), call gtk_main_iteration() repeatedly until there are no more GTK events pending.

Experimental at present, based on https://stackoverflow.com/questions/74209358/unreliable-behaviour-of-gtk-function-gtk-tree-view-scroll-to-cell/75711065

Also need to investigage @cissou8 's report of delays after completion of findstar on images with large numbers of stars, possibly indicating the TreeView is populating itself "behind the scenes" when it doesn't need to.