Restore "expand/collapse on single-click" feature from GTK TreeView

From https://sourceforge.net/p/deadbeef-fb/feature-requests/15/

This would be a great nice-to-have in filebrowser: being able to toggle opening/closing of folders by ONE click on the small triangle. I concede that it would be a bad idea to open them with a single click anywhere the highlight bar is, this should really be left to double-clicking; but when a user clicks on the triangle explicitly (like the [+] on the default file manager of the world's most used OS ;))

This (default) feature got lost when the custom mouseclick handlers were added. It should be possible in GTK to check if the user clicked near the triangle, and expand/collapse the row in this case.