Add a page visually referencing all Control nodes

[Calinou]

Your Godot version: 3.2.3

Issue description:

It's a common source of confusion for new users to be unaware of which Control node they should use to create a given GUI element. Nodes like SpinBox are especially difficult to find for people not used to GUI development.

We should have a page featuring something similar to a "control gallery" where individual GUI elements are represented on an image, outlined and labeled. Something like this (but with nodes outlined and labeled):

The project used to generate this control gallery could also be added to https://github.com/godotengine/godot-demo-projects, since it would be a good showcase for GUI node usage in general.

Edit: Demo PR opened: godotengine/godot-demo-projects#555

See https://wolthera.info/2020/12/xmpp-client-in-godot/.