SwDoc::CopyFlyInFlyImpl: handle textboxes

When the RES_CNTNT of a fly format (used as a textbox) changes, we also
have to update the draw format as well.

CppunitTest_sw_ooxmlimport's testMissingPath is a reproducer for this
problem: the shape (having a textbox) is anchored inside a table, and
the table gets converted into a TextFrame. Without this fix then the
shape still refers to the content before moving, leading to a crash on
loading the document.

Change-Id: I16e2e49dc718f2a4cb00d02513af3dca37b2a65a