[enh] category select warning message

searx/static/default/less/style.less

 	clear: both;
 }
 
+#categories_container {
+    position: relative;
+}
+
 @media screen and (max-width: @results-width) {
 
 	#results {
 		color: @color-settings-return-font;
 	}
 }
+
+.hidden {
+    opacity: 0;
+    overflow: hidden;
+    font-size: 0.8em;
+    position: absolute;
+    bottom: -20px;
+    width: 100%;
+    text-position: center;
+    background: white;
+    transition: opacity 1s ease;
+}
+
+#categories_container:hover .hidden {
+    transition: opacity 1s ease;
+    opacity: 0.8;
+}

searx/templates/default/categories.html

     <div id="categories_container">
 {% for category in categories %}
         <div class="checkbox_container">
-             <input type="checkbox" id="checkbox_{{ category|replace(' ', '_') }}" name="category_{{ category }}" {% if category in selected_categories %}checked="checked"{% endif %} /><label for="checkbox_{{ category|replace(' ', '_') }}">{{ _(category) }}</label>
+             <input type="checkbox" id="checkbox_{{ category|replace(' ', '_') }}" name="category_{{ category }}" {% if category in selected_categories %}checked="checked"{% endif %} /><label for="checkbox_{{ category|replace(' ', '_') }}" class="tooltips">{{ _(category) }}</label>
         </div>
 {% endfor %}
+    <div class="hidden">{{ _('Click on the magnifier to perform search') }}</div>
     </div>
 </div>