get Table container on cmdInsertHeaderRow

git-svn-id: svn+ssh://svn.abisource.com/svnroot/abiword/branches/gsoc2013tables@33386 bcba8976-2d24-0410-9c9c-aab3bd5fdfd6
implementation of cmdRemoveHeaderRow

git-svn-id: svn+ssh://svn.abisource.com/svnroot/abiword/branches/gsoc2013tables@33388 bcba8976-2d24-0410-9c9c-aab3bd5fdfd6