Implement basic virtual methods support

[bilelmoussaoui]

The intention behind this PR is not to add code generation for subclassing code but it does a good step forward.

What the PR does currently:

• Parse virtual-methods from the gir file
• Add configuration options for [object.virtual_method]
• Embed the docs for virtual methods
• Working docs links witth gi-docgen

Before

After

TODO:

• Add more tests for parsing config options
• Document the new config options
• Test it with gtk-rs-core/gstreamer-rs

Parts of #1319

[bilelmoussaoui]

In the latest commits, I went ahead and added docs support for class methods.

Before

After

[bilelmoussaoui]

This should be ready for a proper review now. Can't do much about testing with gtk-rs-core as it is still blocked by the gir rustdoc feature changes.

[bilelmoussaoui]

Tested with gstreamer, works great.

[sdroege]

Looks good to me apart from the naming things

[bilelmoussaoui]

let us land this then :-)