Polish the lua data model.

[actboy168]

This pr continues the work of #146, adding arrays, structures and event callbacks. In addition, it is allowed to continue to register variables and callbacks after constructing the data-model, for example

local options = rmlui.contexts["main"]:OpenDataModel("options", {
    options_changed = false
})
options.graphics = 'ok' -- register new var `graphics `

[mikke89]

Very cool, thank you! I'll take a closer look at it soon.

[mikke89]

Thanks again! These changes are very much welcome. My suggestions are just minor tweaks for code consistency.

I noticed one new issue though: In the LuaInvaders options menu, if you click accept, and then open the menu again, the saved options are not correctly loaded and there is a warning message.

The issue seems to be that the checkbox elements in the data-for loop are constructed after the load event, so in lines 79-80 in options.rml the elements are not found. I suggest changing these lines to modify the data model directly:

	Options.datamodel.audios[1].checked = (options['reverb'] == 'true')
	Options.datamodel.audios[2].checked = (options['3d'] == 'true')

[mikke89]

Thank you!