Layer groups

[dsizzle]

As the number of layers increases in a complex image, it's useful to be able to group them somehow into "folders". This feels complicated to implement. Maybe a good first step is to re-do the Layer window to use a BOutlineListView?

Layer groups need to be:

• expandable/collapsible
• disabled/enabled as a group
• movable as a group
• deletable as a group
• layers can be dragged into and out of groups
• does a layer group get its own blend mode??
• stretch goal, if this isn't enough - layer group drag-n-drop between ArtPaint files.

[humdingerb]

Not much to add, but

• duplicatable as a group
• having a thumbnail of the combined contained layers

Groups having their own blend mode sounds like a good idea.