C#: Implement tooltips for Signals and Properties in the inspector.

[magian1127]

required .csproj <GenerateDocumentationFile>True</GenerateDocumentationFile>

    /// <summary>
    /// tip text
    /// </summary>
    [Export]
    string s1 = "AAAA";

[geekley]

Would it be possible to reuse this implementation for documentation of GDShader uniform fields? They are shown in the inspector, and it would be really useful for sharing shaders; e.g. on sites like https://godotshaders.com

[van800]

If this gets approved, it looks like worth setting GenerateDocumentationFile in the Godot net sdk.

[mhilbrunner]

Removing the documentation label, as while this PR deals with documentation generation/display, this is more suited for the dotnet team to handle :)