skip timer event if write_mutex is locked

git-svn-id: svn://localhost/project_picapica/trunk@303 fb96df88-216d-11de-830c-f3023f12d8ac
implemented FileTransferDialog and added "Send File" menu item in contactlistwidget

git-svn-id: svn://localhost/project_picapica/trunk@304 fb96df88-216d-11de-830c-f3023f12d8ac