theme: Use tabular figures where appropriate

In places where numbers appear in columns (like the calendar widget) or
where changing numbers would result in labels jiggling around due to
small width changes (like the clock), use the newly added
font-feature-settings support to request tabular figures.

https://gitlab.gnome.org/GNOME/gnome-shell/issues/34

data/theme/gnome-shell-sass/_common.scss

@@ -733,6 +733,7 @@ StScrollBar {
   transition-duration: 500ms;
   font-weight: bold;
   height: 1.86em;
+  font-feature-settings: "tnum";
 
   &.unlock-screen,
   &.login-screen,
@@ -958,6 +959,7 @@ StScrollBar {
       padding: 0.1em;
       margin: 2px;
       border-radius: 1.4em;
+      font-feature-settings: "tnum";
       &:hover,&:focus { background-color: lighten($bg_color,5%); }
       &:active,&:selected {
         color: lighten($selected_fg_color,5%);
@@ -1867,6 +1869,7 @@ StScrollBar {
 .screen-shield-clock-time {
   font-size: 72pt;
   text-shadow: 0px 2px 2px rgba(0,0,0,0.4);
+  font-feature-settings: "tnum";
 }
 
 .screen-shield-clock-date {