Create imgui edge windows without declaring a class

Sometimes I just want to quickly prototype a slider and it's a lot of boilerplate to make an `EdgeGUI` subclass.

Maybe something like this:

```python
fig = fpl.Figure()

@fig.add_gui(location="right", title="window title", size=300)
def gui():
    if imgui.button("bah"):
        ...
    
```

related #847 
