In a docx section, any header/footer which is not defined in the
section is considered to be linked to the previous section.
In the Word UI this is shown as an option named "Link to Previous",
and editing the header/footer in either section affects both sections.

LO imports this by copying the header/footer contents from the previous
section, but it did not handle the case where a section had different
headers for left pages vs right pages.

Fix this, which involved changing the mechanism used to detect
whether a section has defined a certain header/footer (which unlinks
it from the previous section):
rather than trying to figure it out from the converted LO page styles,
explicitly track which headers/footers have been defined using boolean
member variables on the SectionPropertyMap instance.

Conflicts:
	sw/qa/extras/ooxmlimport/ooxmlimport.cxx

Change-Id: Ic43a867356c2cd5df09d39f2a3ddefa584b6765c