Fixed VARIANT_ENUM/BITFIELD_CAST to show the correct names

[DmitriySalnikov]

This PR changes how Enums and Bitfields are registered, and also corrects the names of registered enums. Breaks compatibility.

Previously VARIANT_ENUM_CAST could be written in 3 ways:

VARIANT_ENUM_CAST(Example, Example::Constants);
VARIANT_ENUM_CAST(Example, Constants);
VARIANT_ENUM_CAST(, Constants);

The first way was used throughout the generated API. At the same time, VARIANT_BITFIELD_CAST could only be written like this:

VARIANT_BITFIELD_CAST(Example, Example::Flags);

This would not be a problem if such a way of writing would not lead to an incorrect display of enums in the editor.

I changed it to be the same as in Godot itself. Now VARIANT_ENUM_CAST and VARIANT_BITFIELD_CAST can be written like this:

VARIANT_ENUM_CAST(Example::Constants);
VARIANT_BITFIELD_CAST(Example::Flags);
// Or like this for enums outside of classes
VARIANT_ENUM_CAST(Constants);
VARIANT_BITFIELD_CAST(Flags);

Like in Godot, the enum_qualified_name_to_class_info_name function is now used to convert Example::Constants -> Example.Constants.
Added tests.

Actually, if it's very important to keep VARIANT_ENUM_CAST(m_class, m_enum) and VARIANT_BITFIELD_CAST(m_class, m_enum), then I can only leave fixes for VARIANT_BITFIELD_CAST, but I prefer the current version.

[akien-mga]

Thanks!