[SUG][PL] Support for CTRL+key and CTRL+wheel shortcuts

[Guigui]

I love my mousewheel ! And while playing I am undecided wether to use it for the zoom in/out (default iirc), or for the frame stepping forward/backward. So why not use it for both ?

Ideally I think it'd be nice to be able to achieve the following settings :

* mousewheel up = zoom in
* mousewheel down = zoom out (this is already the case by default iirc)

* CTRL + mousewheel up = chosen number of frames step forward
* CTRL + mousewheel down = chosen number of frames step backward