Make page layout lookup code more robust when handling invalid docs

This prevents a null exception being thrown if the requested paragraph
has no details about the associated page style. As a second fallback,
if no style called "Standard" exists, this will fall back to the first
available master-page defined in the ODT.
Clarified ImageController function name
Remove duplicate code in OdfCanvas for finding the master page element
Cope with non-cm image dimensions

Style-related operations are slowly being moved towards px as the
default unit to improve consistency. Standardising getContentSize to
work in px rather than cm provides better re-use of CSSUnits as well.
Ignore test-generated ODTs
Provide better fallback mechanisms for Formatting.getContentSize

getContentSize will now always return a valid page size, falling back
to it's own hardcoded format if no valid one is found for the document
Merge pull request #463 from peitschie/master-page-nullfix

Make page layout lookup code more robust when handling invalid docs