JTree expand and collapse icons are hardcoded in WebTreeUI

[mgarin]

Description
JTree expand and collapse icons are hardcoded in WebTreeUI:

    @NotNull
    @Override
    public Icon getExpandedIcon ()
    {
        return tree.isEnabled () ? Icons.squareMinus : Icons.squareMinusDisabled;
    }

    @NotNull
    @Override
    public Icon getCollapsedIcon ()
    {
        return tree.isEnabled () ? Icons.squarePlus : Icons.squarePlusDisabled;
    }

They should instead be configurable via methods in WebTree or the UI implementation.

Additional information
Based on issue mentioned in question: #702