Improve measure example ux

- Pass click events on measure tooltips through to the map
- Put the tooltip for  the currently drawn geometry on top
- Disable text selection in the tooltips so clicking to draw does not select

examples/measure.css

@@ -7,6 +7,8 @@
   opacity: 0.7;
   white-space: nowrap;
   font-size: 12px;
+  cursor: default;
+  user-select: none;
 }
 .ol-tooltip-measure {
   opacity: 1;