[xcb/xsettings] Add support for byte swapping

The XSettings protocol is not endian neutral. Instead it holds
information about endianness in the first byte. It uses the same
convention as X11/X.h does.
So far byte order handling was missing leading to nasty crashes
when byte order between clients setting and reading XSettings
differed. This patch fixes this.
Using the X11/X.h conventions seems to be an 'established standard',
this piece is missing from the Xsettings specifications. Therefore
this fix may introduce spurious regressions as other Xsettings
'providers' may use a different convention. To detect this and
to avoid crashes the fix also adds checks to avoid reading past
the end of the of the Xsettings data blob. If problems are
encountered: warn and bail.

Change-Id: If8acb23cca2478369633129af2d99e122a84cede
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>