gtk4: Use GtkPicture for left panel indicator icons
Turn gtk4-client left panel indicator icons from GtkImage to GtkPicture. Those work better, and this gets us rid of last use of some GtkImage handling code, which we can (later?) clean out.
Turn gtk4-client left panel indicator icons from GtkImage to GtkPicture. Those work better, and this gets us rid of last use of some GtkImage handling code, which we can (later?) clean out.