Customise mouse wheel/trackpad pad/seek/zoom behaviour

[mifi]

Ideas from #553 #1212 #1859:

• modifier + wheel: seek 1 frame
• modifier + wheel: seek keyframe
• modifier + wheel: seek interval X seconds (customisable?)
• modifier + wheel: seek interval Y seconds (customisable? how many variants?)
• UI button/toggle to switch between variants Interface issues/suggestions #1859 (comment)

Problem: It might be hard to map a "wheel" event to a single consistent unit (see #1859 (comment))

Alternatively (easier) just allow configuring multiple intervals with different modifiers.