fdo#72163 after updating m_xComposer, command facets are not dirty anymore

Else we dispose m_xComposer too eagerly; still used by m_pCacheSet.

Change-Id: I205488465c19a356534df17b8a5e9a20ce6766c9
fdo#72163 belts and suspenders

1) Do not dispose m_xComposer, might still be used by our m_pCache

2) Do not create a new m_xComposer if the previous one will do, so
   that we do not gratiously use a different one than our m_pCache.

Change-Id: I6540c035c9159017c694b36e676721ec3e42db51