Timeline: scroll faster with mouse wheel

[rpallai]

var yDelta = wheel.angleDelta.y / 120 * 100

I don't like this fixed multiplier but even so it's a good start and works well for me.

[rpallai]

The new patch doesn't mess up horizontal wheel scroll of oversized textFields.

[rpallai]

It doesn't mess cursor shape over the timeline.

[rpallai]

It follows the previous approach but provides a more complete implementation.

[rpallai]

It seemed like a low-hanging fruit but it isn't.

The previous approach required all content elements to tile the message with mouse areas which isn't sustainable. Instead of this tiling I decided to use of "fallback" areas at the highest level. It's easier, more complete and transparent for content widgets.

The newly introduced type called TimelineMouseArea will be used more extensivly in my behaviour changing patches in the future. This is why I don't avoid this new type here.

[rpallai]

PR #606 contains this along with other depending mouse handling improvements. I pushed as a whole to save time on review cycles.