Allow system wide shortcut customization

General:

Qt defines a couple of Shortcuts. Some of them are externally consistent (like ctrl+C,...), some are not externally consistent (like Alt+Mousewheel for horizontal scrolling)

Suggestion:

Enable the possibility to customize externally inconsistent shortcuts systemwide (which is probably almost everything except for Ctrl+C/X/V).

Benefit:

On each OS a Qt app is used, it feels more unified and integrated into the system.

( I think it's a good idea to allow shortcuts to be configured by OSs and not by toolkits ).

Specific:

Qt defines Alt+Mousewheel for horizontal scrolling. However, this collides on Ubuntu systems, because

1) they use Shift+Mousewheel for horizontal scrolling.

2) they define Alt+Mousewheel for desktop zoom and window dragging/resizing.

References:

https://bugs.kde.org/show_bug.cgi?id=431554