Add "permanently Shift down" functionality

[vbHeiko]

Just one example:

[Press F2] "permanently Shift down" use Scroll Wheel = horizontally-scroll eBay images (next image/previous image)

[Press F2 again] "release permanently Shift down"

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With this Shift-toggle-function it is possible to:

1. perform a horizontal web page scroll
2. scroll eBay photos (forward/backward)