Use GtkSettings 'gtk-xft-dpi' property to keep track of DPI changes

Use that property to find the screen's resolution for font handling
instead of gdk_screen_get_resolution(), and also connect a callback
to notify::gtk-xft-dpi so that we can update the font size when the
DPI value changes for the screen, and not just on startup.

https://bugzilla.gnome.org/show_bug.cgi?id=744796