Tree: support wide cell renderer

[DevCharly]

This PR adds support for wide tree cell renderers, which occupy (nearly) full width of tree.

Left screenshot shows bounds of wide cells renderers (red rectangles).
Right screenshot shows default (non-wide) cell bounds.

This gives you more flexibility and you can use whole row width to show some extra information. (see issue #922)
E.g. an icon for some rows:

How to enable

For a single tree:

tree.putClientProperty( FlatClientProperties.TREE_WIDE_CELL_RENDERER, true );

For all trees in app use properties file:

Tree.wideCellRenderer = true