Add new option to "merge only visible" on Layer > Flatten option

[dacap]

If there are hidden layers, the Layer > Flatten should show an check-box/radio-button to keep those layers:

(*) Merge visible layers & keep hidden layers
( ) Merge visible layers & don't delete hidden layers

Another option in case there are selected layers:

...
(*) Merge selected layers

[dacap]

This issue + #1157 will give us the possibility to select multiple (non-adjacent) layers and merge them.

More discussion here: https://community.aseprite.org/t/wish-flatten-selected-layers/163

[jjhaggar]

I'd love to have this feature on Aseprite! Also I think that to be able to "Flatten a Layer Group, keeping its name" would be also an interesting (and probably related) feature :)

[dacap]

Flatten will merge selected layers in the timeline if the range is enabled (the thick border in the timeline) or merge all layers if there is no thick border in the timeline. We'll be able to flat just one group too and a new menu option Layer > Flatten Visible Layers was added too.