Add manual generation support

[Eastrall]

This PR introduces features based on user suggestions, code cleaning and improves usage experience.

Manual generation

Suggested in #26

When creating a UXML document, the Rosalina automatic binding generator is disable. To enable it, you must right-click on the UXML file > Rosalina > Properties...

A new editor window opens next to the Inspector window and you can enable the code generation for this file by checking the Enable checkbox.
Additionnaly, you can now select which generator you want to use for the UXML file.

Generic usage

Suggestd in #26

Bindings generators now use Q<T>() method instead of (T)Q().

public void InitializeDocument()
{
    InformationLabel = Root?.Q<Label>("InformationLabel");
    ConfirmButton = Root?.Q<Button>("confirm-button");
    CancelButton = Root?.Q<Button>("CancelButton");
    TitleLabel = Root?.Q<Label>("TitleLabel");
    BodyContainer = Root?.Q<VisualElement>("BodyContainer");
}

Custom component

Suggested in #25

Added new UXML generator for custom components. Custom components are built by passing a VisualElement instance representing the root element of the component.

// <auto-generated>
using UnityEngine.UIElements;

public partial class CustomComponent
{
    public Label Title { get; private set; }

    public Button NewButton { get; private set; }

    public VisualElement Root { get; }

    public CustomComponent(VisualElement root)
    {
        Root = root;
        Title = Root?.Q<Label>("Title");
        NewButton = Root?.Q<Button>("NewButton");
    }
}

Then if you add your custom component to a document, you will get the following generated code:

// <auto-generated>
using UnityEngine;
using UnityEngine.UIElements;

public partial class SampleDocument
{
    [SerializeField]
    private UIDocument _document;

    public Label InformationLabel { get; private set; }

    public CustomComponent CustomLabel { get; private set; }

    public VisualElement Root => _document?.rootVisualElement;

    public void InitializeDocument()
    {
        InformationLabel = Root?.Q<Label>("InformationLabel");
        // Create a new CustomComponent instance using the `Q<T>()` method.
        CustomLabel = new CustomComponent(Root?.Q<VisualElement>("CustomLabel"));
    }
}

Settings improvement

Setting file was originaly placed in /Assets/Rosalina folder. It has been moved to the /ProjectSettings folder and doesn't belong to the assets anymore.
Also, you can view which files is acutally in the Rosalina is being processed by the AssetProcessor for bindings generation: