Use tabular numbers in file transfer dialog

[cassidyjames]

Problem

When a file is being transferred, the string showing the progress jitters all over the place due to the differing widths of the numbers.

progress.mp4

Proposal

We use tabular numbers in the file transfer dialog (or really, anywhere we're showing filesizes, probably). (Idea from @alatiera@mastodon.social.)

Prior Art

GNOME Web seems to use tabular numbers in the download popover to prevent the string from jumping around.

[alatiera]

If any newcomer is interested, what you'd need to do is apply the a tnum=1 font-feature attribute to the GtkLabel through gtk_label_set_attributes

The equivalent gtkbuilder xml is this.

      <object class="GtkLabel" id="current_time">
        <property name="label">0:00</property>
        <attributes>
          <attribute name="font-features" value="tnum=1"/>
        </attributes>
      </object>

[cassidyjames]

Since we don't use GtkBuilder XML… here's an equivalent PR to the date/time indicator: https://github.com/elementary/wingpanel-indicator-datetime/pull/246/files

Basically, wrap the relevant string in <span font_features='tnum'>%s</span>.