[Releasing] Printing changes for 5.3

[John Layt]

Hi,

We've been working on a stack of changes to QtPrintSupport [1], but in spite 
of everyone's best reviewing and testing efforts there's still a couple of 
blocker issues outstanding.  I'd like to still get a few parts of it into 5.3 
so peoples efforts are not wasted, and we're in a better state to finish the 
changes in 5.4.

What I plan to do regardless before we fork:
* New QMarginsF class approved and merged
* Some small Windows fixes merged
* The new QPA class QPlatformPrintDevice and the platform implementations 
merged, but not actually used anywhere.  This allows us to ask people with 5.3 
built / installed to run the manual tests against their print devices to see 
if the new code works before we depend on it in 5.4.

I'll carry on working on the blocker issues over the weekend to see if we can 
resolve them, if not I'll assess if any more of the new code can be merged or 
if we defer it all to 5.4.

Cheers!

John.