Keyboard shortcut clashes

Reported by @Klaus Cinibulk :
When in Retouch mode, Shift + Mouse wheel isn’t working for me, when the mouse cursor is inside the retouch window. I have to use the curser keys instead, which is a bit inconvenient.

On a German keyboard the [], {} and \ aren’t easily usable without breaking your fingers. So I re-mapped “Brush size down” to “Shift+A” and “Brush size up” to “Shift+S”. While “Shift+A” is working, “Shift+S” isn’t. As it seems, it’s because the “S” is used for “Show source frame (hold). And that’s what actually happens, the “Shift” seems to be ignored.