Add slow transition FX

[DedeHai]

A new effect that is purely made for very slow transitions, up to 255 minutes.
It also allows to call a preset after the transition is done by setting intensity slider to the preset number to be applied after the transition has finished.
Using segment layering, this also allows it to be a "mask" although that takes a bit of tinkering with the preset json.

One scenario possible with this effect would be this:

• start with a preset using this FX, say fully black, set the next preset to one with the same FX but that one at full brightness at a warm white with a transition time of 60 minutes
• the LEDs will transition from black to warm white over 60 mintues, at the end it sets another preset "cold white"
• the "cold white" preset transitions to full bright daylight white in another 60 minutes

The FX only transitions palettes (colors if palette 0 or any color derived palette is used), brightness slider transitions normally i.e. fast. If a color/palette is changed mid transition, it starts a new transition towards that color with the current state as a starting point so transitions never jump.

p.s.
I also remvoed the return value from Segment::loadPalette()` as the target palette is passed by reference and the return value is never even used anymore.

Fixes #5375