Merge pull request #27 from waveform80/master

Small update to add shift+scroll to zoomed graphs
Add reversed axis self test
Axis cleanup
Rename function
Added fill-to-edge capability

Added Edge property to FillAbove and FillBelow and relabelled them to
Fill1 and Fill2 (as per Jeremy's suggestion on the veusz-discuss list
[1]). However there seems to be some bug with drawing an XY line graph
with a filled error component.

The relabelling also makes the hide-error-fill options a bit
meaningless...

[1] https://mail.gna.org/public/veusz-discuss/2013-01/msg00010.html