Add placeholders to elements with no value

Issue: Samsung#266
Problem: 1. User doesn't know what to write in form inputs
Solution: Add placeholders with example values

Signed-off-by: Kornelia Kobiela <k.kobiela@samsung.com>

design-editor/src/panel/property/attribute/templates/attribute-box-model.html

@@ -18,11 +18,11 @@
 </li>
 <li class="closet-attribute-common-border">
     <span class="closet-attribute-common-label-long">box shadow</span>
-    <input type="text" name="boxShadow" class="native-key-bindings closet-attribute-common-textbox long-controls">
+    <input type="text" name="boxShadow" class="native-key-bindings closet-attribute-common-textbox long-controls" placeholder="10px 10px 5px #000">
 </li>
 <li class="closet-attribute-common-border">
     <span class="closet-attribute-common-label">border</span>
-    <input type="text" name="borderWidth" class="native-key-bindings closet-attribute-common-textbox">
+    <input type="text" name="borderWidth" class="native-key-bindings closet-attribute-common-textbox" placeholder="10px">
     <select name="borderStyle" class="closet-attribute-common-textbox closet-attribute-common-border-style">
         <option>none</option>
         <option>solid</option>
@@ -39,5 +39,5 @@
 </li>
 <li class="closet-attribute-common-border">
     <span class="closet-attribute-common-label-long">border radius</span>
-    <input type="text" name="borderRadius" class="native-key-bindings closet-attribute-common-textbox">
+    <input type="text" name="borderRadius" class="native-key-bindings closet-attribute-common-textbox long-controls" placeholder="10px or 50%">
 </li>