Show appropriate tooltips for disabled undo/redo buttons

Implementing this cross-browser requires using a `.disabled`
class rather than the `disabled` property. No browsers except
Opera dispatch mouse events on disabled buttons.

Fixes #620.

css/app.css

@@ -273,7 +273,12 @@ button.active {
     cursor:url(../img/cursor-pointing.png) 6 1, auto;
 }
 
-button.active:not([disabled]) {
+button.disabled {
+    background: #6c6c6c;
+    cursor: auto;
+}
+
+button.active:not([disabled]):not(.disabled) {
     background: #6bc641;
 }
 
@@ -450,8 +455,8 @@ button[disabled] .icon.browse     { background-position:   0px -40px;}
 button[disabled] .icon.add-point  { background-position: -20px -40px;}
 button[disabled] .icon.add-line   { background-position: -40px -40px;}
 button[disabled] .icon.add-area   { background-position: -60px -40px;}
-button[disabled] .icon.undo       { background-position: -80px -40px;}
-button[disabled] .icon.redo       { background-position: -100px -40px;}
+button.disabled  .icon.undo       { background-position: -80px -40px;}
+button.disabled  .icon.redo       { background-position: -100px -40px;}
 button[disabled] .apply.icon      { background-position: -120px -40px;}
 button[disabled] .save.icon       { background-position: -140px -40px;}
 button[disabled] .close.icon      { background-position: -160px -40px;}

js/id/ui.js

@@ -91,14 +91,14 @@ iD.ui = function (context) {
 
         undo_buttons.append('button')
             .attr({ id: 'undo', 'class': 'col6' })
-            .property('disabled', true)
+            .classed('disabled', true)
             .html("<span class='undo icon'></span><small></small>")
             .on('click.editor', history.undo)
             .call(undo_tooltip);
 
         undo_buttons.append('button')
             .attr({ id: 'redo', 'class': 'col6' })
-            .property('disabled', true)
+            .classed('disabled', true)
             .html("<span class='redo icon'><small></small>")
             .on('click.editor', history.redo)
             .call(undo_tooltip);
@@ -209,13 +209,13 @@ iD.ui = function (context) {
             }
 
             limiter.select('#undo')
-                .property('disabled', !undo)
-                .attr('data-original-title', hintprefix('⌘ + Z', undo))
+                .classed('disabled', !undo)
+                .attr('data-original-title', hintprefix('⌘ + Z', undo || t('nothing_to_undo')))
                 .call(refreshTooltip);
 
             limiter.select('#redo')
-                .property('disabled', !redo)
-                .attr('data-original-title', hintprefix('⌘ + ⇧ + Z', redo))
+                .classed('disabled', !redo)
+                .attr('data-original-title', hintprefix('⌘ + ⇧ + Z', redo || t('nothing_to_redo')))
                 .call(refreshTooltip);
         });
 

locale/en.js

@@ -141,6 +141,9 @@ locale.en = {
     "zoom-in": "Zoom In",
     "zoom-out": "Zoom Out",
 
+    nothing_to_undo: "Nothing to undo.",
+    nothing_to_redo: "Nothing to redo.",
+
     "browser_notice": "This editor is supported in Firefox, Chrome, Safari, Opera, and Internet Explorer 9 and above. Please upgrade your browser or use Potlatch 2 to edit the map.",
 
     "layer_settings": "Layer Settings",