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

[netmindz]

Not tried it, but sounds exactly what one of my customers wanted from my lamps

[DedeHai]

it also was requested somewhere before, I saw it more than once but could not find the references.
Its not the perfect solution and the "call preset after fade" is a bit of a hack or at least not strictly future proof as an FX basically has no business calling presets but I am not aware of anything breaking if it does it.
this probably needs some more testing on systems that use mqtt or sync or other means of external communication. I tested only on a confined device and there it works quite well.

In a nutshell this is the "sunrise FX" but instead of a fixed sunrise pattern, it allows to use any palette/color as a starting point and as a destination. I am not sure it is very easy to use though: if you just call it once it does nothing but initialize, so you have to use it either in a playlist or in a "chain of presets"