regtest: Use diff.png extension for image diff files

So that they are recognized as images
regtest: Add create-report command to generate html report of test results
regtest: Reduce the noise of the default output when running tests

Show permanent information only about failed tests, without the details
about the failing pages. Previous verbose output is available passing
--verbose command line output.
gxps-document: Fix a crash in documents containing nested FixedDocument

It's a bug in the document itself, but we shouldn't crash anyway.

https://bugzilla.gnome.org/show_bug.cgi?id=685741
Add helper function to check if a given path is part of the archive
Add gxps_zip_archive_iter_next()

Helper function to make it easier to iterate the archive and reduces
some duplicated code.
Add support for interleaved archives
Fix parsing of real numbers containing e[+-]number
Add API to get Core Propoerties of a XPS file
test-gxps: Add a button to the toolbar to show a core properties window
Fix the build with libpng16

Include string.h explicitly in gxps-png-writer.c since png.h no longer
includes it.
Cleanup: remove GXPSArchiveEntry

https://bugzilla.gnome.org/show_bug.cgi?id=698022
Cleanup: remove unused variable

https://bugzilla.gnome.org/show_bug.cgi?id=698022
gxps-page: Fix parsing of dash array when it has trailing or leading whitespaces

Strip the string before splitting it.

https://bugzilla.gnome.org/show_bug.cgi?id=715023
gxps-page: Make gxps_dash_array_parse fail for an odd number of dashes

Spotted by Lukas Lueg.

https://bugzilla.gnome.org/show_bug.cgi?id=715023