gui: Use tabular numbers in transfer labels

Use the tabular numbers (tnum) OpenType feature of PangoAttrFontFeature
to prevent numbers from jumping around as transfer progress changes.

This means that "11" occupies the same space as "44" instead of jumping
around.

See: GNOME/gtk#41
See: GNOME/gnome-builder#1027
3 jobs for fix-wrapping-notifications-1027 in 27 minutes and 43 seconds (queued for 1 second)