Add minimum size for slider (#905)

Author: wpkelso

lib/Styles/Gtk/Slider.scss

@@ -1,4 +1,5 @@
 slider {
+    $diameter: rem(16px);
     @include control;
 
     border: none;
@@ -9,6 +10,10 @@ slider {
         0 0 0 1px $border-color,
         shadow(2);
 
+    // 16px - border
+    min-height: calc(#{$diameter} - 1px);
+    min-width: calc(#{$diameter} - 1px);
+
     &:backdrop {
         box-shadow:
             highlight(),