glib-mkenums: allow optional 'since' tag

The glib-mkenums program allows generating code to handle enums/flags
with very different purposes. One of its purposes could be generating
per-enum/flag methods to be exposed in a library API, and while doing
that, it would be nice to have a way to specify in which API version
the enum/flag was introduced, so that the same version could be shown
in the generated API methods.

E.g. From the following code:

    /**
     * QmiWmsMessageProtocol:
     * @QMI_WMS_MESSAGE_PROTOCOL_CDMA: CDMA.
     * @QMI_WMS_MESSAGE_PROTOCOL_WCDMA: WCDMA.
     *
     * Type of message protocol.
     *
     * Since: 1.0
     */
    typedef enum { /*< since=1.0 >*/
        QMI_WMS_MESSAGE_PROTOCOL_CDMA  = 0x00,
        QMI_WMS_MESSAGE_PROTOCOL_WCDMA = 0x01
    } QmiWmsMessageProtocol;

The template would allow us to generate a method documented like this,
including the Since tag with the value given in the mkenums 'since' tag.

    /**
     * qmi_wms_message_protocol_get_string:
     * @val: a QmiWmsMessageProtocol.
     *
     * Gets the nickname string for the #QmiWmsMessageProtocol specified at @val.
     *
     * Returns: (transfer none): a string with the nickname, or %NULL if not found. Do not free the returned value.
     * Since: 1.0
     */
    const gchar *qmi_wms_message_protocol_get_string (QmiWmsMessageProtocol val);

Signed-off-by: Aleksander Morgado <aleksander@aleksander.es>

docs/reference/gobject/glib-mkenums.xml

@@ -195,6 +195,14 @@ Specifies the word separation used in the <function>*_get_type()</function>
 function. For instance, <literal>/*&lt; underscore_name=gnome_vfs_uri_hide_options &gt;*/</literal>.
 </para></listitem>
 </varlistentry>
+<varlistentry>
+<term><literal>since</literal></term>
+<listitem><para>
+Specifies the version tag that will be used to substitute the <literal>@enumsince@</literal>
+keyword in the template, useful when documenting methods generated from the enums
+(e.g. <literal>Since: @enumsince@</literal>). (Since: 2.66)
+</para></listitem>
+</varlistentry>
 </variablelist>
 
 <para>The following options can be specified per value definition:</para>
@@ -219,7 +227,7 @@ typedef enum /*&lt; skip &gt;*/
 {
   PREFIX_FOO
 } PrefixThisEnumWillBeSkipped;
-typedef enum /*&lt; flags,prefix=PREFIX &gt;*/
+typedef enum /*&lt; flags,prefix=PREFIX,since=1.0 &gt;*/
 {
   PREFIX_THE_ZEROTH_VALUE,	/*&lt; skip &gt;*/
   PREFIX_THE_FIRST_VALUE,