add DoxygenLayout.xml file

`DoxygenLayout.xml` is picked up by doxygen automatically (as `Doxyfile`)
and its template can be generated by `doxygen -l DoxygenLayout.xml`.

after that, the following tweaks were done:
- remove all xml-parts but header (other defaults are fine so far)
- rename "Modules" header to "Constants" (there are no modules :)
- remove useless subitems from "Classes", eg. by name only shows as "d"
- reorder tabs so that "Classes" becomes the first one

deltachat-ffi/DoxygenLayout.xml

@@ -0,0 +1,38 @@
+<doxygenlayout version="1.0">
+  <!-- Generated by doxygen 1.8.20 -->
+  <!-- Navigation index tabs for HTML output -->
+  <navindex>
+    <tab type="mainpage" visible="yes" title=""/>
+    <tab type="classes" visible="yes" title="">
+      <tab type="classlist" visible="no" title="" intro=""/>
+      <tab type="classindex" visible="no" title=""/>
+      <tab type="hierarchy" visible="no" title="" intro=""/>
+      <tab type="classmembers" visible="no" title="" intro=""/>
+    </tab>
+    <tab type="modules" visible="yes" title="Constants" intro="Here is a list of constants:"/>
+    <tab type="pages" visible="yes" title="" intro=""/>
+    <tab type="namespaces" visible="yes" title="">
+      <tab type="namespacelist" visible="yes" title="" intro=""/>
+      <tab type="namespacemembers" visible="yes" title="" intro=""/>
+    </tab>
+    <tab type="interfaces" visible="yes" title="">
+      <tab type="interfacelist" visible="yes" title="" intro=""/>
+      <tab type="interfaceindex" visible="$ALPHABETICAL_INDEX" title=""/>
+      <tab type="interfacehierarchy" visible="yes" title="" intro=""/>
+    </tab>
+    <tab type="structs" visible="yes" title="">
+      <tab type="structlist" visible="yes" title="" intro=""/>
+      <tab type="structindex" visible="$ALPHABETICAL_INDEX" title=""/>
+    </tab>
+    <tab type="exceptions" visible="yes" title="">
+      <tab type="exceptionlist" visible="yes" title="" intro=""/>
+      <tab type="exceptionindex" visible="$ALPHABETICAL_INDEX" title=""/>
+      <tab type="exceptionhierarchy" visible="yes" title="" intro=""/>
+    </tab>
+    <tab type="files" visible="yes" title="">
+      <tab type="filelist" visible="yes" title="" intro=""/>
+      <tab type="globals" visible="yes" title="" intro=""/>
+    </tab>
+    <tab type="examples" visible="yes" title="" intro=""/>
+  </navindex>
+</doxygenlayout>