fix(slider): wrong thumb label cursor

Uses the `grab` cursor when hovering over an active slider's thumb and thumb labels.

Fixes #3778.

Author: crisbeto

src/lib/slider/slider.scss

@@ -1,4 +1,5 @@
 @import '../core/style/variables';
+@import '../core/style/vendor-prefixes';
 
 
 // This refers to the thickness of the slider. On a horizontal slider this is the height, on a
@@ -98,6 +99,8 @@ $mat-slider-focus-ring-size: 30px !default;
 }
 
 .mat-slider-thumb {
+  @include cursor-grab;
+
   position: absolute;
   right: -$mat-slider-thumb-size / 2;
   bottom: -$mat-slider-thumb-size / 2;
@@ -113,6 +116,8 @@ $mat-slider-focus-ring-size: 30px !default;
 }
 
 .mat-slider-thumb-label {
+  @include cursor-grab;
+
   display: none;
   align-items: center;
   justify-content: center;
@@ -237,6 +242,10 @@ $mat-slider-focus-ring-size: 30px !default;
   .mat-slider-thumb-label {
     display: none;
   }
+
+  .mat-slider-thumb, .mat-slider-thumb-label {
+    cursor: default;
+  }
 }