Improve combobox option visibility

(closes openstreetmap#4761)

This improves the usability of the combobox when it appears over
other rows of fields (e.g. raw tag editor)
1. shrink size on left/right to provide more separation from content beneath
2. adjust box shadow for slightly more visibility
3. shrink size of first option by one pixel, so that the combobox
   options will never perfectly line up with rows that they cover

css/80_app.css

@@ -1883,7 +1883,7 @@ input[type=number] {
 div.combobox {
     z-index: 9999;
     display: none;
-    box-shadow: 0 0 10px 0 rgba(0,0,0,.1);
+    box-shadow: 0 4px 10px 1px rgba(0,0,0,.2);
     margin-top: -1px;
     background: white;
     max-height: 120px;
@@ -1896,7 +1896,7 @@ div.combobox {
 .combobox a {
     display: block;
     padding: 5px 10px;
-    border-top:1px solid #ccc;
+    border-top: 1px solid #ccc;
     text-overflow: ellipsis;
     white-space: nowrap;
     overflow: hidden;
@@ -1909,6 +1909,7 @@ div.combobox {
 
 .combobox a:first-child {
     border-top: 0;
+    padding: 4px 10px;
 }
 
 .combobox-caret {

modules/lib/d3.combobox.js

@@ -265,8 +265,8 @@ export function d3combobox() {
             var rect = node.getBoundingClientRect();
 
             wrapper
-                .style('left', rect.left + 'px')
-                .style('width', rect.width + 'px')
+                .style('left', (rect.left + 5) + 'px')
+                .style('width', (rect.width - 10) + 'px')
                 .style('top', rect.height + rect.top + 'px');
         }