Nodegraphqt

- New nodes palette widget 40
https://jchanvfx.github.io/NodeGraphQt/api/html/custom_widgets.html#nodes-palette

- implemented ``NodeGraph.set_pipe_colision()`` function. 206

see docs: https://jchanvfx.github.io/NodeGraphQt/api/html/graph.html#NodeGraphQt.NodeGraph.set_pipe_collision

- Addressed issue 202

Fix property list widget delegate bg color.
Fix bug with selecting items above other items 205

- Ports can now be locked with (``Port.lock()``, ``Port.unlock()``) to prevent new pipe connections and current pipes to be disconnected.
- Refactored node auto layout logic into the ``NodeGraph`` class.
- ``NodeGraph.auto_layout_nodes(nodes=None, down_stream=True, start_nodes=None)``
- Added `BackdropNode.wrap_nodes(nodes=[])` function to fit backdrop size to nodes.
- Fixed `BackdropNode.auto_size()` function as it was not written with an undo block.
- Cleaned up drag & drop functions.
example drag & drop strings :
- ``'ngqt://path/to/node/session.graph'``
- ``'ngqt::node:com.nodes.MyNode1;node:com.nodes.MyNode2'``
- Removed vendor module.
- closed issue 192
- closed issue 195
- bug fixes.

- node factory attributes now instanced for multiple graphs 188
- new ``BaseNode.add_custom_widget()`` function for embedding custom widgets into a BaseNode object.
example: https://jchanvfx.github.io/NodeGraphQt/api/html/examples/ex_node.html#embedding-custom-widgets