added stack orientation (xy:default,yz,zy) as a field to stack
coordinate transfer from and into project coordinates respect this
orientation
pure stack navigation with re-oriented static stacks working
added vertical and horizontal middle line to stack
moved orientation field from stack to projectstack relation as it
specifies the orientation of a stack relative to a project
even better help and disclaimer icons, still not happy with the slightly
magenta blue (why is that so?  I am generating them with 0000ff blue,
0 transparency!)
Finally managed to get the icons with correct color.
It required to get rid of the undesired sRGB profile that GIMP
automatically assigns during PNG export.  That's possible with ImageMagick

convert -strip icon.png icon.png
fix navigation in zoom-levels < 0 (no integer rounding for x and y)
fixed selector tool to respect stack scale and orientation
added helper methods to stack.js for obtaining the limiting coordinates
of the bounding box of the current slice and for transferring box
coordinates from stack to project space.
Fix some whitespace errors
Merge branch 'orthoslices'
Add user profile setting to toggle display of reference lines

The merge of the orthoslice branch introduced reference lines for the
stack display. A vertical and a horizontal bar is shown, meeting at the
center of the view. This commits adds a user profile option to enable or
disable the display of these reference lines.
Add checkbox to navigator to toggle the display of reference lines