Middle-mouse button scrolling in the Source pane

[DavidKinder]

In the HTML panes, you can hold down the middle mouse button (or scroll wheel) and move the mouse to scroll. It would be nice to have this in the Source pane, too.

This is (somewhat) implemented in MFC, and there is a modified version of this already in use for the Skein window. That could be improved by having a better icon image for when this mode activates - currently it's just a crudely drawn circle with arrows, with no anti-aliasing.

[DavidKinder]

Commit f986798 tidies up the icon logic for the skein window. Now the middle mouse scrolling icon (the "anchor pan" icon) is antialiased.