Add "events" category to wxEvent-derived classes docs where missing

If a wxEvent-derived class documentation lacks "events" category,
the class will not appear in the Events class list.

The "events" category is still missing in several event classes
(e.g., palette events or wxRibbonToolBarEvent), but those lack
actual documentation aside from just methods declarations anyway.

interface/wx/aui/auibar.h

@@ -12,7 +12,7 @@
     See also @ref overview_aui.
 
     @library{wxaui}
-    @category{aui}
+    @category{aui,events}
   */
 enum wxAuiToolBarStyle
 {

interface/wx/html/htmlwin.h

@@ -559,7 +559,7 @@ wxEventType wxEVT_HTML_LINK_CLICKED;
     @endEventTable
 
     @library{wxhtml}
-    @category{html}
+    @category{html,events}
 */
 class wxHtmlLinkEvent : public wxCommandEvent
 {
@@ -592,7 +592,7 @@ public:
     @endEventTable
 
     @library{wxhtml}
-    @category{html}
+    @category{html,events}
 */
 class wxHtmlCellEvent : public wxCommandEvent
 {

interface/wx/propgrid/propgrid.h

@@ -1332,7 +1332,7 @@ public:
     @endEventTable
 
     @library{wxpropgrid}
-    @category{propgrid}
+    @category{propgrid,events}
 */
 class wxPropertyGridEvent : public wxCommandEvent
 {

interface/wx/socket.h

@@ -479,7 +479,7 @@ public:
     @endEventTable
 
     @library{wxnet}
-    @category{net}
+    @category{net,events}
 
     @see wxSocketBase, wxSocketClient, wxSocketServer
 */

interface/wx/treelist.h

@@ -902,6 +902,9 @@ public:
 /**
     Event generated by wxTreeListCtrl.
 
+    @category{ctrl,events}
+
+
     @since 2.9.3
  */
 class wxTreeListEvent : public wxNotifyEvent

interface/wx/webrequest.h

@@ -788,7 +788,7 @@ public:
     @since 3.1.5
 
     @library{wxnet}
-    @category{net}
+    @category{net,events}
 
     @see wxWebRequest
 */