Add graph doc and graph dtd

doc/graph.t2t

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+
+== Graph view ==
+
+The dtd that describe the xml for a graph view is stored in trytond/ir/ui/graph.dtd.
+
+* XML description:
+
+- tag "graph":
+
+    - "type": vbar, hbar, line, pie
+
+    - "string": the name of the graph
+
+    - "background": an hexaecimal value for the color of the background
+
+    - "color": the main color
+
+    - "legend": a boolean to specify if the legend must be display
+
+- tag "x", "y":
+
+    Describe the field that must be used for axis.
+    "x" must contain only one tag "field" and "y" must at least one but may contain many.
+
+- tag "field":
+
+    - "name": the name of the field on the object to use
+
+    - "string": allow to override the string that comes from the object
+
+    - "key": can be used to distinguish fields with the same name but that are different with domain
+
+    - "domain": a string that is evaluate with the object value as context. If the result is true the field value is added to the graph otherwise not
+
+    - "fill": defined if the graph must be fill
+
+
+* Example:
+
+  <graph string="Invoice by date" type="vbar">
+    <x>
+        <field name="invoice_date"/>
+    </x>
+    <y>
+        <field name="total_amount"/>
+    </y>
+  </graph>

trytond/ir/ui/graph.dtd

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+<!ELEMENT graph (x, y)>
+<!ATTLIST graph type (vbar|hbar|line|pie) "vbar">
+<!ATTLIST graph string CDATA "Unknown">
+<!ATTLIST graph background CDATA "#f5f5f5">
+<!ATTLIST graph color CDATA "blue">
+<!ATTLIST graph legend (0|1) "1">
+<!ELEMENT x (field)>
+<!ELEMENT y (field+)>
+<!ELEMENT field EMPTY>
+<!ATTLIST field name CDATA #REQUIRED>
+<!ATTLIST field string CDATA #IMPLIED>
+<!ATTLIST field key CDATA #IMPLIED>
+<!ATTLIST field domain CDATA #IMPLIED>
+<!ATTLIST field fill (0|1) #IMPLIED>