lokdocview: Use a thread pool for most LOK calls

This is a thread pool with just single thread because LOK is
single threaded; using multiple threads in this case would be
useless.

Primary reason we are using a thread pool here is to avoid the
overhead in spawning a new thread for each LOK call.

Change-Id: Ibbfdb7cb0a8ef9f07bcc659e65ce8997716aa245

include/LibreOfficeKit/LibreOfficeKitGtk.h

@@ -81,8 +81,8 @@ gboolean                       lok_doc_view_get_edit               (LOKDocView*
 
 /// Posts the .uno: command to the LibreOfficeKit.
 void                           lok_doc_view_post_command           (LOKDocView* pDocView,
-                                                                    const char* pCommand,
-                                                                    const char* pArguments);
+                                                                    const gchar* pCommand,
+                                                                    const gchar* pArguments);
 
 float                          lok_doc_view_pixel_to_twip          (LOKDocView* pDocView,
                                                                     float fInput);