[Example] Add shapes drawing example application

[miroiu]

📝 Description of the Change

This PR adds a new application example that demonstrates how to implement some common functionalities:

• implementing undo/redo for common actions (#101)
• implementing resizable nodes
• using decorators for drawing a node toolbar and users (cursors) in a collaborative environment (#64)
• using the mouse location in editor coordinates to draw shapes by clicking and dragging
• using editor commands to zoom, fit nodes to screen, and bring locations into view (locations of other users in this case)
• using gestures to make the editor read-only, or enter drawing mode
• customizing the template for ItemContainer and Connector
• using the step connection
• using the editor without including a theme in App.xaml

Keybinds:

• Delete selection: Delete
• Undo and redo changes: Ctrl+Z and Ctrl+Y or Ctrl+Shift+Z
• Zoom in and out: Ctrl+Plus or Ctrl+Minus
• Fit to screen: Shift+Home
• Remove connection: Right Mouse Button or Alt+Left Mouse Button

Closes #33 Related #116

Credits: https://reactflow.dev/examples/nodes/shapes

[BAndysc]

I really like this demo app! It is just beautiful! I couldn't resist and ported it to Avalonia, the best part - it is possible to run this demo in a browser :D https://cca7896a.nodify-avalonia.pages.dev/ it can be sluggish compared to the desktop version, but hey, it runs everywhere :)

[miroiu]

I'm impressed. It's working well in the browser, despite the minor visual glitch of the grid pattern when zooming out.

I really like this demo app! It is just beautiful!

Thanks! I like it too, but the design is not mine. Here's the original app: https://reactflow.dev/examples/nodes/shapes

[BAndysc]

Thanks! I like it too, but the design is not mine. Here's the original app: https://reactflow.dev/examples/nodes/shapes

Ah, I see, still I am impressed how well you recreated the design and how nice Nodify works for this example :) Great job