Print: support margin saving for older Qt versions

Seems like QMarginsF and QPageLayout were added quite
recently - in Qt5.3!

We attempt to support older versions by using something
found in the Qt4.7 documentation.

Patch also fixes small ordering issue where top / left
margins were swapped.

Signed-off-by: Lubomir I. Ivanov <neolit123@gmail.com>
Signed-off-by: Robert C. Helling <helling@atdotde.de>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Add .txt as dive log file

Add the .txt to the list of all dive log files as this is the file we
expect to be opened for Poseidon Mk6 log files.

Signed-off-by: Miika Turkia <miika.turkia@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
First stab at README and ReleaseNotes for 4.3

It's never too early to start...

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>