Add VariantResource class

[Xrayez]

Allows to store built-in core data types as resources. Can be edited via editor inspector and stored on disk!

Array

Dictionary

Closes godotengine/godot-proposals#1740.

Color

Closes godotengine/godot-proposals#1734.

And other types. It means that the value is changed dynamically whenever you change the type property, and in some cases allows you to convert the old value to a new type.

[Xrayez]

Added various PropertyInfo properties as properties in VariantResource. Allows to customize the value property of the resource itself dynamically:

Hints are listed from PropertyHint enum in the engine, which should be added to Godot: godotengine/godot#43211.

Hint string are used to configure the property's editor behavior:

Most essential hint strings are documented in Godot at global scope documentation.

PropertyUsageFlags are not so trivial to expose to be user-friendly, but still it should be there for completeness.

Modifying String properties in the VariantResource doesn't update the editor inspector for some reason, reported a bug in Godot godotengine/godot#43238, I may try to fix the issue there. The workaround is to switch the history of edited objects back and worth with the arrows above to force update the editor inspector that way.

That said, I think this is good enough to merge!

[Xrayez]

Implemented resource previews as described in godotengine/godot-proposals#1734 (comment).

[Xrayez]

Generating default icon resource previews for each Variant compatible type now:

Except for Color currently, because it makes perfect sense to display a color there.

The bool could be displayed as a "switch", "bulb" etc which could encode whether the value is turned on/off right from the filesystem dock.

Integer, float and string values could be rendered I think, but that's only theory and not sure whether it's possible/worth it.

[Xrayez]

So, there are actually "GuiChecked" and "GuiUnchecked" editor icons which can be reused to preview bool resources:

An appropriate icon was found with the help of https://github.com/Xrayez/godot-editor-icons-previewer plugin. 🙂

Which, may be worth integrating in Goost at some point, currently /editor is empty...