[ADD] web: Add a new widget to attach documents on a record

Purpose
=======

Add the possibility to easily attach a document on a record. That could be used for to attach receipts on expense for example.

Specification
=============

Add a new widget (AttachDocument) to allow attach document from mobile/web directly on record form

After attaching document it will call action given on widget, that action will be a method name on given model

addons/web/static/src/xml/base.xml

@@ -1351,4 +1351,20 @@
     </div>
 </t>
 
+<t t-name="AttachDocument">
+    <button class="btn btn-sm btn-default o_attachment_button">
+        <span class="o_attach_document"><t t-esc="widget.node.attrs.string"/></span>
+        <span class="hide">
+            <t t-call="HiddenInputFile">
+                <t t-set="fileupload_id" t-value="widget.fileuploadID"/>
+                <t t-set="fileupload_action" t-translation="off">/web/binary/upload_attachment</t>
+                <t t-set="multi_upload" t-value="true"/>
+                <input type="hidden" name="model" t-att-value="widget.res_model"/>
+                <input type="hidden" name="id" t-att-value="widget.res_id"/>
+                <input type="hidden" name="session_id" t-att-value="widget.getSession().session_id"/>
+            </t>
+        </span>
+    </button>
+</t>
+
 </templates>