NEW: axis padding property

[Emmanuel Pacaud <emmanuel pacaud univ-poitiers fr>]

Hi,

I've just commited a patch that adds a padding property to axes, only
for X and Y axes for now. I guess it could be extended to circular axes
too.

It allows user to leave some space between plot area and axis line.

Here's a sample output:

http://emmanuel.pacaud.free.fr/screenshots/gnumeric/gnumeric-axis-padding.png
http://emmanuel.pacaud.free.fr/screenshots/gnumeric/gnumeric-axis-padding-pref.png

        Emmanuel.