Add custom widget for "percent" field

so that storage can use "traditional" (0.3612) format but UI can use
"human-friendly" format (36.12 %)

tailbone/templates/deform/percentinput.pt

@@ -0,0 +1,25 @@
+<span tal:define="name name|field.name;
+                  css_class css_class|field.widget.css_class;
+                  oid oid|field.oid;
+                  mask mask|field.widget.mask;
+                  mask_placeholder mask_placeholder|field.widget.mask_placeholder;
+                  style style|field.widget.style;
+                  autocomplete autocomplete|field.widget.autocomplete|'off';
+"
+      tal:omit-tag="">
+    <input type="text" name="${name}" value="${cstruct}" 
+           tal:attributes="class string: form-control ${css_class or ''};
+                           style style;
+                           autocomplete autocomplete;
+                           "
+           id="${oid}"/>
+    %
+    <script tal:condition="mask" type="text/javascript">
+      deform.addCallback(
+         '${oid}',
+         function (oid) {
+            $("#" + oid).mask("${mask}", 
+                 {placeholder:"${mask_placeholder}"});
+         });
+    </script>
+</span>