support for GNUStep extensions

[imz]

Example (a GNUStep extension to the old-style plist format; generated by sogo-tool backup):

    SOGoFirstDayOfWeek = <*I0>;
    SOGoFirstWeekOfYear = January1;
    SOGoGravatarEnabled = <*I0>;

The <*I0> has turned out to be a GNUStep extension (adding the same data types as in the new-style XML-based plist) meaning "integer"; cf. NSPropertyList.h:

 *   <term>[NSNumber]</term>
 *   <desc>
 *     Number objects were not traditionally allowed in <em>property lists</em>
 *     but were added when the XML format was introduced.  GNUstep provides
 *     an extension to the traditional <em>property list</em> format to
 *     support number objects, but older code will not read
 *     <em>property lists</em> containing this extension.<br />
 *     Numbers are stored in a variety of formats depending on their values.
 *     <list>
 *       <item>boolean ... either <code>&lt;*BY&gt;</code> for YES or
 *         <code>&lt;*BN&gt;</code> for NO.<br />
 *         In XML format this is either <code>&lt;true /&gt;</code> or
 *         <code>&lt;false /&gt;</code>
 *       </item>
 *       <item>integer ... <code>&lt;*INNN&gt;</code> where NNN is an
 *         integer.<br />
 *         In XML format this is <code>&lt;integer&gt;NNN&lt;integer&gt;</code>
 *       </item>
 *       <item>real ... <code>&lt;*RNNN&gt;</code> where NNN is a real
 *         number.<br />
 *         In XML format this is <code>&lt;real&gt;NNN&lt;real&gt;</code>
 *       </item>
 *     </list>
 *   </desc>

And see other extensions described there.