SwTextBoxHelper::findTextBox: take a const SwFrmFmt

Change-Id: I531a5864f61ab6c9bfcfc44f7fcacef58ad78489
SwTextBoxHelper::findTextBoxes: return a set of const pointers

Change-Id: I8e3ea996959bad72e6d15dc9c980ea9d18d31fb3
fdo#82191 sw::DocumentLayoutManager: copy textbox content of draw formats

The SwFmtCntnt, i.e. the content of the draw format was already copied,
but that's only a pointer to the real content: instead duplicate the
real contents on copy&paste, that's how we copy fly frames as well.

Change-Id: I42475e356aaa1c54c08fb23a6a395d1726e5f33e