String::Len was used in a non-bool context here

...so 79f52d249fe043e6ec54be6ffa0c71a748920394 "convert sw/source/ui/inc/e*.hxx
from String to OUString" should have converted it to call OUString::getLength,
not OUString::isEmpty.

Change-Id: Ie5633a1ff87094eb22179f14aab4b1508735a747
Change various SwTransferable functions to return bool

...which required to add an explicit channel to return
SWTRANSFER_GRAPHIC_INSERTED from SwTransferable::_PasteFileName.

Change-Id: I6d1401830f82dc44b37879ebdd0345cff14eecfa