Small UI improvements, require gtk+ >= 3.13
Replace deprecated gtk_* functions

gtk_application_add_accelerator() has been deprecated in gtk+ 3.13, use
gtk_application_set_accels_for_action() instead.
contact widget: Some small layout changes
neuland-contact-widget.h: Fix structs

They were accidentally set to GtkListBoxRow instead of GtkBox.
contact widget: Switch to GtkGrid

This makes aligning easier.