Reword/slightly rename options

Also some general code cleanup.

Change-Id: Iaefd3a3694af9b369f83484714bc21e13104561e

gr-image-diff-tool/gr-image-diff-tool.html

diff --git a/gr-image-diff-tool/gr-image-diff-tool.html b/gr-image-diff-tool/gr-image-diff-tool.html
index 18c34f1..e85a260 100644
--- a/gr-image-diff-tool/gr-image-diff-tool.html
+++ b/gr-image-diff-tool/gr-image-diff-tool.html
@@ -53,10 +53,10 @@
       }
     </style>
     <div id="header">
-      <h3>Image Diff</h3>
+      <h3>Image diff</h3>
       <select value="{{_observeMode::change}}" id="dropdown">
-        <option value="resemble">Resemble Mode</option>
-        <option value="opacity">Opacity Mode</option>
+        <option value="resemble" title="Scale the images to the same size and compute a diff with highlights">Highlight differences</option>
+        <option value="opacity" title="Overlay the new image over the old and use an opacity control to view the differences">Onion skin</option>
       </select>
     </div>
     <div class="diffmode">

gr-opacity-diff-mode/gr-opacity-diff-mode.html

diff --git a/gr-opacity-diff-mode/gr-opacity-diff-mode.html b/gr-opacity-diff-mode/gr-opacity-diff-mode.html
index ee79d1d..bd0db70 100644
--- a/gr-opacity-diff-mode/gr-opacity-diff-mode.html
+++ b/gr-opacity-diff-mode/gr-opacity-diff-mode.html
@@ -22,7 +22,7 @@
       }
       .wrapper {
         box-shadow: 0 1px 3px rgba(0, 0, 0, .3);
-        margin: 1em 0em;
+        margin: 1em 0;
       }
       img {
         display: block;
@@ -47,7 +47,7 @@
       #controlsBox {
         display: flex;
         justify-content: space-between;
-        margin: 0 1em;
+        margin: 0 .5em;
         min-width: 32em;
         width: 100%;
       }

gr-resemble-diff-mode/gr-resemble-diff-mode.html

diff --git a/gr-resemble-diff-mode/gr-resemble-diff-mode.html b/gr-resemble-diff-mode/gr-resemble-diff-mode.html
index 056e979..edc6a93 100644
--- a/gr-resemble-diff-mode/gr-resemble-diff-mode.html
+++ b/gr-resemble-diff-mode/gr-resemble-diff-mode.html
@@ -109,7 +109,7 @@
         <button
             id="fullscreen"
             on-tap="_handleFullScreen">
-          View full screen
+          View full sized
         </button>
       </div>
     </div>