XML support

[XuNeo]

Introduce the problem

Describe the UI in XML, parse and create UI in luavgl.

XML can be preprocessed before running to a standard lua file. Or it can be parsed and executed in run time.

XML can contain lua script, just like js script in html.

Proposal

No response

[cnjhb]

XML is completely unnecessary. XML is primarily used to achieve declarative layouts, which Lua can accomplish. We should implement a method for declarative layouts using Lua.

[XuNeo]

I originally thought it would be great to align with LVGL XML, so generated XML can be used in lua directly too.
luavgl now supports declarative in certain level. I hope this is easy to use.