Adapt to lvgl's property API

[XuNeo]

Fix #11

Make full use of lvgl's property API for binding language.
For simple properties, luavgl will call lv_obj_property_ API directly.

luavgl_toproperty and luavgl_pushproperty is used to convert value between lua and lv_property_t.

New feature

Now the property can be accessed directly in nature way. E.g

obj.x = 123;
print("obj  x:", obj.x)

obj.align = lvgl.ALIGN.CENTER
obj.align = {
    type = lvgl.ALIGN.CENTER
    x_ofs = 10,
    y_ofs = 10,
}

[XuNeo]

Not all changes have been upstreamed, thus switched to https://github.com/XuNeo/lvgl/tree/luavgl-property