mouse scroll shouldn't affect settings drop down lists and any other thing if it is not selected

[ciros88]

extension of #272

[ciros88]

currently affected (2.0.60.Alpha):

• libopenmpt "stereo separation"
• libsidplayfb "check for songlength update"
• uade "silence timeout"

however, the fix shouldn't be element specific (punctual) but should, by default, prevent any preferences element (generic)